GENEVA U.N. human rights investigators have gathered testimony from casualties of Syria's civil war and medical staff indicating that rebel forces have used the nerve agent sarin, one of the lead investigators said on Sunday. 
 
 The United Nations independent commission of inquiry on Syria has not yet seen evidence of government forces having used chemical weapons, which are banned under international law, said commission member Carla Del Ponte. 
 
 "Our investigators have been in neighboring countries interviewing victims, doctors and field hospitals and, according to their report of last week which I have seen, there are strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas, from the way the victims were treated," Del Ponte said in an interview with Swiss-Italian television. 
 
 "This was use on the part of the opposition, the rebels, not by the government authorities," she added, speaking in Italian. 
 
 Del Ponte, a former Swiss attorney-general who also served as prosecutor of the International Criminal Tribunal for the former Yugoslavia, gave no details as to when or where sarin may have been used. 
 
 The Geneva-based inquiry into war crimes and other human rights violations is separate from an investigation of the alleged use of chemical weapons in Syria instigated by U.N. Secretary-General Ban Ki-moon, which has since stalled. 
 
 President Bashar al-Assad's government and the rebels accuse each another of carrying out three chemical weapon attacks, one near Aleppo and another near Damascus, both in March, and another in Homs in December. 
 
 The civil war began with anti-government protests in March 2011. The conflict has now claimed an estimated 70,000 lives and forced 1.2 million Syrian refugees to flee. 
 
 The United States has said it has "varying degrees of confidence" that sarin has been used by Syria's government on its people. 
 
 President Barack Obama last year declared that the use or deployment of chemical weapons by Assad would cross a "red line". 
 
 (Reporting by Stephanie Nebehay; Editing by Tom Pfeiffer) ||||| Syrian rebels shot down a military helicopter in the country's east, killing eight government troops on board a day after opposition forces entered a sprawling military air base in the north, activists said Monday. 
 
 This citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, shows planes on the ground at the Kweiras military air base... (Associated Press) 
 
 In the past months, rebels fighting to topple President Bashar Assad have frequently targeted military aircraft and air bases in an attempt to deprive his regime of a key weapon used to target opposition strongholds and reverse rebel gains in the 2-year-old conflict. 
 
 The Britain-based Syrian Observatory for Human Rights on Monday posted a video online showing several armed men standing in front of the wreckage. One of the fighters in the footage says it's a helicopter that the rebels shot down late Sunday in the eastern province of Deir el-Zour, along Syria's border with Iraq. 
 
 As the man speaks, the camera shifts to a pickup truck piled with bodies. The fighter is then heard saying that all of Assad's troops who were aboard the helicopter were killed in the downing. He says Islamic fighters of the Abu Bakr Sadiqq brigade brought down the helicopter as it was taking off from a nearby air base in the provincial capital of Deir el-Zour. 
 
 The Observatory, which relies on a network of activists on the ground, said eight troops were killed. 
 
 On Sunday, rebels occupied parts of a military air base in northern Syria after days of fighting with government troops who were defending the sprawling facility near the border with Turkey for months, the Observatory said. 
 
 Assad's warplanes were pounding rebel positions inside the Mannagh air base Monday as clashes between rebels and government forces raged on, the Observatory said, adding there was an unknown number of casualties on both sides. 
 
 The day before, rebels moved deep into the air base despite fire from government warplanes, capturing a tank unit inside the base and killing the base's commander, Brig. Gen. Ali Salim Mahmoud, according to another activists group, the Aleppo Media Center. 
 
 The fighting came hours after Israeli warplanes struck areas in and around the Syrian capital, setting off a series of explosions as they targeted a shipment of highly accurate, Iranian-made guided missiles believed to be bound for Lebanon's Hezbollah militant group, officials and activists said. 
 
 The attack, the second in three days and the third this year, signaled a sharp escalation of Israel's involvement in Syria's civil war. Syrian state media reported that Israeli missiles on Sunday struck a military and scientific research center near Damascus and caused casualties. 
 
 The Syrian conflict started with largely peaceful protests against President Bashar Assad's regime in March 2011, but eventually turned into a civil war that has killed more than 70,000 people according to the United Nations. 
 
 More than one million Syrians have fled their homes during the fighting and sought shelter in the neighboring countries such as Jordan, Lebanon and Turkey. Millions of others have been displaced inside Syria. ||||| Washington (CNN) Russian involvement in the 4½-year-old Syrian civil war seems to be escalating, with NATO's secretary general confirming Tuesday a second incursion by Russian planes into Turkish airspace and saying Russian ground troops were in Syria as well. 
 
 Damascus skies erupt with anti-aircraft fire as the US and its allies launch an attack on Syria's capital early on April 14, 2018. US President Donald Trump announced airstrikes in retaliation for Syria's alleged use of chemical weapons. Trump says the strikes are part of a sustained military response, in coordination with France and the United Kingdom. 
 
 Damascus skies erupt with anti-aircraft fire as the US and its allies launch an attack on Syria's capital early on April 14, 2018. US President Donald Trump announced airstrikes in retaliation for Syria's alleged use of chemical weapons. Trump says the strikes are part of a sustained military response, in coordination with France and the United Kingdom. 
 
 Bodies lie on the ground in the rebel-held city of Douma, Syria, on April 8, 2018. According to activist groups, helicopters dropped barrel bombs filled with toxic gas on Douma, which has been the focus of a renewed government offensive that launched in mid-February. The Syrian government and its key ally, Russia, vehemently denied involvement and accused rebel groups of fabricating the attack to hinder the army's advances and provoke international military intervention. 
 
 A child receives medical treatment after a village was attacked in the rebel-held Eastern Ghouta region on February 25, 2018. Several people were treated for exposure to chlorine gas, opposition groups said, as airstrikes and artillery fire from the regime continued. CNN was unable to independently verify claims that chlorine was used as a weapon. 
 
 A child receives medical treatment after a village was attacked in the rebel-held Eastern Ghouta region on February 25, 2018. Several people were treated for exposure to chlorine gas, opposition groups said, as airstrikes and artillery fire from the regime continued. CNN was unable to independently verify claims that chlorine was used as a weapon. 
 
 A member of the Syrian pro-regime forces fires a machine gun as a comrade holds his feeding ammunition belt on November 11, 2017. It was during an advance toward rebel-held positions west of Aleppo. 
 
 A member of the Syrian pro-regime forces fires a machine gun as a comrade holds his feeding ammunition belt on November 11, 2017. It was during an advance toward rebel-held positions west of Aleppo. 
 
 Residents of the war-torn city of Douma break their Ramadan fast on June 18, 2017. 
 
 Residents of the war-torn city of Douma break their Ramadan fast on June 18, 2017. 
 
 Members of the UN Security Council raise their hands on April 12, 2017, as they vote in favor of a draft resolution that condemned the reported use of chemical weapons in Syria. 
 
 Members of the UN Security Council raise their hands on April 12, 2017, as they vote in favor of a draft resolution that condemned the reported use of chemical weapons in Syria. 
 
 This photo, provided by the activist Idlib Media Center, shows dead children after a suspected chemical attack in the rebel-held city of Khan Sheikhoun on April 4, 2017. Dozens of people were killed, according to multiple activist groups. The United States responded a few days later by launching between 50-60 Tomahawk missiles at a Syrian government airbase. US officials said the base was home to warplanes that carried out the chemical attack. Syria has repeatedly denied it had anything to do with the attack. 
 
 This photo, provided by the activist Idlib Media Center, shows dead children after a suspected chemical attack in the rebel-held city of Khan Sheikhoun on April 4, 2017. Dozens of people were killed, according to multiple activist groups. The United States responded a few days later by launching between 50-60 Tomahawk missiles at a Syrian government airbase. US officials said the base was home to warplanes that carried out the chemical attack. Syria has repeatedly denied it had anything to do with the attack. 
 
 Arabic writing that reads "some day we will return" is seen on a bus window as civilians evacuate Aleppo on December 15, 2016. The evacuations began under a new ceasefire between rebels and pro-government forces. 
 
 Arabic writing that reads "some day we will return" is seen on a bus window as civilians evacuate Aleppo on December 15, 2016. The evacuations began under a new ceasefire between rebels and pro-government forces. 
 
 Smoke rises after an airstrike in Aleppo on October 4, 2016. 
 
 Smoke rises after an airstrike in Aleppo on October 4, 2016. 
 
 This still image, taken from a video posted by the Aleppo Media Center, shows a young boy in an ambulance after an airstrike in Aleppo, Syria, on August 17, 2016. It took nearly an hour to dig the boy, identified as Omran Daqneesh, out from the rubble, an activist told CNN. The airstrike destroyed his home, where he lived with his parents and two siblings. Director of the Aleppo Media Center Yousef Saddiq said Omran's 10-year-old brother, Ali, died from his injuries. 
 
 This still image, taken from a video posted by the Aleppo Media Center, shows a young boy in an ambulance after an airstrike in Aleppo, Syria, on August 17, 2016. It took nearly an hour to dig the boy, identified as Omran Daqneesh, out from the rubble, an activist told CNN. The airstrike destroyed his home, where he lived with his parents and two siblings. Director of the Aleppo Media Center Yousef Saddiq said Omran's 10-year-old brother, Ali, died from his injuries. 
 
 Members of a Syrian opposition group attack the headquarters of al-Assad regime forces in the Aleppo villages of Nubul and al-Zahraa on February 12, 2016. 
 
 Members of a Syrian opposition group attack the headquarters of al-Assad regime forces in the Aleppo villages of Nubul and al-Zahraa on February 12, 2016. 
 
 A sandstorm blows over damaged buildings in the rebel-held area of Douma, east of Damascus, on September 7, 2015. 
 
 A sandstorm blows over damaged buildings in the rebel-held area of Douma, east of Damascus, on September 7, 2015. 
 
 A refugee carries mattresses as he re-enters Syria from Turkey on June 22, 2015, after Kurdish People's Protection Units regained control of the area around Tal Abyad, Syria, from ISIS. 
 
 A refugee carries mattresses as he re-enters Syria from Turkey on June 22, 2015, after Kurdish People's Protection Units regained control of the area around Tal Abyad, Syria, from ISIS. 
 
 A Syrian child fleeing the war gets lifted over fences to enter Turkish territory illegally near a border crossing at Akcakale, Turkey, on June 14, 2015. 
 
 A Syrian child fleeing the war gets lifted over fences to enter Turkish territory illegally near a border crossing at Akcakale, Turkey, on June 14, 2015. 
 
 Nusra Front fighters inspect a helicopter belonging to pro-government forces after it crashed in the rebel-held Idlib countryside on March 22, 2015. 
 
 Nusra Front fighters inspect a helicopter belonging to pro-government forces after it crashed in the rebel-held Idlib countryside on March 22, 2015. 
 
 Rebel fighters dig caves in the mountains for bomb shelters in the northern countryside of Hama on March 9, 2015. 
 
 Rebel fighters dig caves in the mountains for bomb shelters in the northern countryside of Hama on March 9, 2015. 
 
 Medics tend to a man's injuries at a field hospital in Douma after airstrikes on September 20, 2014. 
 
 Medics tend to a man's injuries at a field hospital in Douma after airstrikes on September 20, 2014. 
 
 Volunteers remove a dead body from under debris after shelling in Aleppo on August 29, 2014. According to the Syrian Civil Defense, barrel bombs are now the greatest killer of civilians in many parts of Syria. The White Helmets are a humanitarian organization that tries to save lives and offer relief. 
 
 Volunteers remove a dead body from under debris after shelling in Aleppo on August 29, 2014. According to the Syrian Civil Defense, barrel bombs are now the greatest killer of civilians in many parts of Syria. The White Helmets are a humanitarian organization that tries to save lives and offer relief. 
 
 Photographs of victims of the Assad regime are displayed as a Syrian army defector known as "Caesar," center, appears in disguise to speak before the House Foreign Affairs Committee in Washington. The July 31, 2014, briefing was called "Assad's Killing Machine Exposed: Implications for U.S. Policy." Caesar, apparently a witness to the regime's brutality, smuggled more than 50,000 photographs depicting the torture and execution of more than 10,000 dissidents. CNN cannot independently confirm the authenticity of the photos, documents and testimony referenced in the report. 
 
 Photographs of victims of the Assad regime are displayed as a Syrian army defector known as "Caesar," center, appears in disguise to speak before the House Foreign Affairs Committee in Washington. The July 31, 2014, briefing was called "Assad's Killing Machine Exposed: Implications for U.S. Policy." Caesar, apparently a witness to the regime's brutality, smuggled more than 50,000 photographs depicting the torture and execution of more than 10,000 dissidents. CNN cannot independently confirm the authenticity of the photos, documents and testimony referenced in the report. 
 
 Rebel fighters execute two men on July 25, 2014, in Binnish, Syria. The men were reportedly charged by an Islamic religious court with detonating several car bombs. 
 
 Rebel fighters execute two men on July 25, 2014, in Binnish, Syria. The men were reportedly charged by an Islamic religious court with detonating several car bombs. 
 
 A giant poster of al-Assad is seen in Damascus on May 31, 2014, ahead of the country's presidential elections. He received 88.7% of the vote in the country's first election after the civil war broke out. 
 
 A giant poster of al-Assad is seen in Damascus on May 31, 2014, ahead of the country's presidential elections. He received 88.7% of the vote in the country's first election after the civil war broke out. 
 
 A US ship staff member wears personal protective equipment at a naval airbase in Rota, Spain, on April 10, 2014. A former container vessel was fitted out with at least $10 million of gear to let it take on about 560 metric tons of Syria's most dangerous chemical agents and sail them out to sea, officials said. 
 
 A US ship staff member wears personal protective equipment at a naval airbase in Rota, Spain, on April 10, 2014. A former container vessel was fitted out with at least $10 million of gear to let it take on about 560 metric tons of Syria's most dangerous chemical agents and sail them out to sea, officials said. 
 
 A man holds a baby who was rescued from rubble after an airstrike in Aleppo on February 14, 2014. 
 
 A man holds a baby who was rescued from rubble after an airstrike in Aleppo on February 14, 2014. 
 
 An injured man is helped following an airstrike in Aleppo's Maadi neighborhood on December 17, 2013. 
 
 An injured man is helped following an airstrike in Aleppo's Maadi neighborhood on December 17, 2013. 
 
 Syrian children wait as doctors perform medical checkups at a refugee center in Sofia, Bulgaria, on October 26, 2013. 
 
 Syrian children wait as doctors perform medical checkups at a refugee center in Sofia, Bulgaria, on October 26, 2013. 
 
 Residents run from a fire at a gasoline and oil shop in Aleppo's Bustan Al-Qasr neighborhood on October 20, 2013. Witnesses said the fire was caused by a bullet from a pro-government sniper. 
 
 Residents run from a fire at a gasoline and oil shop in Aleppo's Bustan Al-Qasr neighborhood on October 20, 2013. Witnesses said the fire was caused by a bullet from a pro-government sniper. 
 
 The UN Security Council passes a resolution September 27, 2013, requiring Syria to eliminate its arsenal of chemical weapons. Al-Assad said he would abide by the resolution. 
 
 The UN Security Council passes a resolution September 27, 2013, requiring Syria to eliminate its arsenal of chemical weapons. Al-Assad said he would abide by the resolution. 
 
 An aerial view shows the Zaatari refugee camp near the Jordanian city of Mafraq on July 18, 2013. 
 
 An aerial view shows the Zaatari refugee camp near the Jordanian city of Mafraq on July 18, 2013. 
 
 Rebels launch a missile near the Abu Baker brigade in Al-Bab, Syria, on January 16, 2013. 
 
 Rebels launch a missile near the Abu Baker brigade in Al-Bab, Syria, on January 16, 2013. 
 
 Syrians look for survivors amid the rubble of a building targeted by a missile in the al-Mashhad neighborhood of Aleppo on January 7, 2013. 
 
 Syrians look for survivors amid the rubble of a building targeted by a missile in the al-Mashhad neighborhood of Aleppo on January 7, 2013. 
 
 A father reacts after the deaths of two of his children in Aleppo on January 3, 2013. 
 
 A father reacts after the deaths of two of his children in Aleppo on January 3, 2013. 
 
 The bodies of three children are laid out for identification by family members at a makeshift hospital in Aleppo on December 2, 2012. The children were allegedly killed in a mortar shell attack that landed close to a bakery in the city. 
 
 The bodies of three children are laid out for identification by family members at a makeshift hospital in Aleppo on December 2, 2012. The children were allegedly killed in a mortar shell attack that landed close to a bakery in the city. 
 
 Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo as fighting continues through the night on December 1, 2012. 
 
 Smoke rises in the Hanano and Bustan al-Basha districts in Aleppo as fighting continues through the night on December 1, 2012. 
 
 An Israeli tank crew sits on the Golan Heights overlooking the Syrian village of Breqa on November 6, 2012. Israel fired warning shots toward Syria after a mortar shell hit an Israeli military post. It was the first time Israel fired on Syria across the Golan Heights since the 1973 Yom Kippur War. 
 
 An Israeli tank crew sits on the Golan Heights overlooking the Syrian village of Breqa on November 6, 2012. Israel fired warning shots toward Syria after a mortar shell hit an Israeli military post. It was the first time Israel fired on Syria across the Golan Heights since the 1973 Yom Kippur War. 
 
 A Syrian rebel walks inside a burnt section of the Umayyad Mosque in Aleppo hours before the Syrian army retook control of the complex on October 14, 2012. 
 
 A Syrian rebel walks inside a burnt section of the Umayyad Mosque in Aleppo hours before the Syrian army retook control of the complex on October 14, 2012. 
 
 Smoke rises over the streets after a mortar bomb from Syria landed in the Turkish border village of Akcakale on October 3, 2012. Five people were killed. In response, Turkey fired on Syrian targets and its parliament authorized a resolution giving the government permission to deploy soldiers to foreign countries. 
 
 Smoke rises over the streets after a mortar bomb from Syria landed in the Turkish border village of Akcakale on October 3, 2012. Five people were killed. In response, Turkey fired on Syrian targets and its parliament authorized a resolution giving the government permission to deploy soldiers to foreign countries. 
 
 Free Syrian Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16, 2012. 
 
 Free Syrian Army fighters are reflected in a mirror they use to see a Syrian Army post only 50 meters away in Aleppo on September 16, 2012. 
 
 A Syrian man carrying grocery bags dodges sniper fire in Aleppo as he runs through an alley near a checkpoint manned by the Free Syrian Army on September 14, 2012. 
 
 A Syrian man carrying grocery bags dodges sniper fire in Aleppo as he runs through an alley near a checkpoint manned by the Free Syrian Army on September 14, 2012. 
 
 Family members mourn the deaths of their relatives in front of a field hospital in Aleppo on August 21, 2012. 
 
 Family members mourn the deaths of their relatives in front of a field hospital in Aleppo on August 21, 2012. 
 
 A Free Syrian Army fighter runs for cover as a Syrian Army tank shell hits a building across the street during clashes in the Salaheddine neighborhood of central Aleppo on August 17, 2012. 
 
 A Free Syrian Army fighter runs for cover as a Syrian Army tank shell hits a building across the street during clashes in the Salaheddine neighborhood of central Aleppo on August 17, 2012. 
 
 Rebel fighters with the Free Syrian Army capture a police officer in Aleppo, Syria, who they believed to be pro-regime militiaman on July 31, 2012. Dozens of officers were reportedly killed as rebels seized police stations in the city. 
 
 Rebel fighters with the Free Syrian Army capture a police officer in Aleppo, Syria, who they believed to be pro-regime militiaman on July 31, 2012. Dozens of officers were reportedly killed as rebels seized police stations in the city. 
 
 Syrian refugees walk across a field in Syria before crossing into Turkey on March 14, 2012. 
 
 Syrian refugees walk across a field in Syria before crossing into Turkey on March 14, 2012. 
 
 Supporters of al-Assad celebrate during a referendum vote in Damascus on February 26, 2012. Opposition activists reported at least 55 deaths across the country as Syrians headed to the polls. Analysts and protesters widely described the constitutional referendum as a farce. "Essentially, what (al-Assad's) done here is put a piece of paper that he controls to a vote that he controls so that he can try and maintain control," a US State Department spokeswoman said. 
 
 Supporters of al-Assad celebrate during a referendum vote in Damascus on February 26, 2012. Opposition activists reported at least 55 deaths across the country as Syrians headed to the polls. Analysts and protesters widely described the constitutional referendum as a farce. "Essentially, what (al-Assad's) done here is put a piece of paper that he controls to a vote that he controls so that he can try and maintain control," a US State Department spokeswoman said. 
 
 Delegates from Arab League member states and Turkey discuss a response to the government's crackdown in Syria on November 16, 2011. 
 
 Delegates from Arab League member states and Turkey discuss a response to the government's crackdown in Syria on November 16, 2011. 
 
 Jamal al-Wadi speaks in Istanbul on September 15, 2011, after an alignment of Syrian opposition leaders announced the creation of a Syrian National Council -- their bid to present a united front against Bashar al-Assad's regime and establish a democratic system. 
 
 Jamal al-Wadi speaks in Istanbul on September 15, 2011, after an alignment of Syrian opposition leaders announced the creation of a Syrian National Council -- their bid to present a united front against Bashar al-Assad's regime and establish a democratic system. 
 
 Syrian children walk over bricks stored for road repairs during a spontaneous protest June 15, 2011, at a refugee camp near the Syrian border in Yayladagi, Turkey. 
 
 Syrian children walk over bricks stored for road repairs during a spontaneous protest June 15, 2011, at a refugee camp near the Syrian border in Yayladagi, Turkey. 
 
 Anti-government protesters demonstrate in Daraa on March 23, 2011. In response to continuing protests, the Syrian government announced several plans to appease citizens. 
 
 Anti-government protesters demonstrate in Daraa on March 23, 2011. In response to continuing protests, the Syrian government announced several plans to appease citizens. 
 
 An injured man lying in the back of a vehicle is rushed to a hospital in Daraa, Syria, on March 23, 2011. Violence flared in Daraa after a group of teens and children were arrested for writing political graffiti. Dozens of people were killed when security forces cracked down on demonstrations. 
 
 An injured man lying in the back of a vehicle is rushed to a hospital in Daraa, Syria, on March 23, 2011. Violence flared in Daraa after a group of teens and children were arrested for writing political graffiti. Dozens of people were killed when security forces cracked down on demonstrations. 
 
 Displaced Syrian residents wait to receive food aid distributed by the UN Relief and Works Agency at the besieged al-Yarmouk camp, south of Damascus, Syria, on January 31, 2014. According to the UN Envoy for Syria, an estimated 400,000 Syrians have been killed since an uprising in March 2011 spiraled into civil war. See how the conflict has unfolded. 
 
 Displaced Syrian residents wait to receive food aid distributed by the UN Relief and Works Agency at the besieged al-Yarmouk camp, south of Damascus, Syria, on January 31, 2014. According to the UN Envoy for Syria, an estimated 400,000 Syrians have been killed since an uprising in March 2011 spiraled into civil war. See how the conflict has unfolded. 
 
 "It's unacceptable, it's dangerous, and it's reckless behavior and it adds to the tensions," Secretary General Jens Stoltenberg told CNN about the incursions into Turkish airspace. 
 
 "We see the violation of the Turkish airspace becomes more important, more dangerous, because it happens in a context where we see more fighting, more Russian military presence in Syria." 
 
 Stoltenberg expressed alarm over how the Russian military grows on several fronts in Syria. 
 
 "So we have seen a substantial military buildup by Russia in Syria, both in the air with the combat planes and air defense systems, but also an increasing number of ground troops," he said. "In addition to that, they have deployed naval assets, a large number of naval assets close to the Syrian shores. And they continue to do so." 
 
 Alleged airspace violations 
 
 The first violation of Turkey's airspace is reported to have happened Saturday. The second was Sunday, officials said. 
 
 Turkey's President Recep Tayyip Erdogan had a warning for Moscow, according to the nation's semiofficial news agency, Anadolu. 
 
 "If Russia loses a friend like Turkey, with which it has conducted many businesses, it will lose a lot," he said. 
 
 NATO chief Jens Stoltenberg condemns Russia's incursion into Turkish airspace. 
 
 In addition to the incursion reports, the Turkish air force said a MiG-29 fighter jet of an unidentified nationality had interfered Monday with eight Turkish F-16s on the Turkey-Syria border. The Turkish air force reported the jet put its planes under a radar lock for four minutes and 30 seconds. 
 
 Stoltenberg rebuked Russian combat aircraft for two alleged violations of Turkey's airspace. 
 
 "We also have seen two of them, two violations of Turkish airspace," he said at a news conference. "Intelligence that we have received provides me with reason to say it doesn't look like an accident." 
 
 Turkey has been a member of NATO since 1952, and alliance rules say that an attack on any of the 28 member states should be treated as an attack on all. 
 
 Stoltenberg said the "substantial buildup of Russian forces in Syria" includes "ground troops in connection with the air base they have." 
 
 Stoltenberg said he doubted that Russia, which has been conducting airstrikes in Syria, was interested primarily in fighting the terrorist group ISIS, also known as ISIL. 
 
 "I'm also concerned that Russia is not targeting ISIL but instead attacking the Syrian opposition and civilians," he said. 
 
 U.S.: Russia moved weapons, troops into Syria 
 
 The Pentagon also is worried about Russia's military activity in Syria. 
 
 The latest U.S. assessment indicates that Russia has moved ground combat weapons and troops to operate them into areas of western Syria where anti-regime forces are, according to two American defense officials. The United States sees the move as Russia "stepping up its ground activity" in Syria to attack those forces, rather than ISIS elements, according to one of the officials. 
 
 But Russian officials deny ramping up military activity. Officials quoted by state media said there would be no ground operation in Syria and -- in contrast to what officials had said earlier -- Russia would try to prevent any "volunteers" from going to Syria. 
 
 The tension between Russia and Turkey, however, appears to be escalating. The two countries are at odds over the civil war in Syria, with Russia backing Syrian President Bashar al-Assad. 
 
 "I discussed the situation in Syria with Foreign Minister Lavrov in New York last week," Stoltenberg said, referring to Russian Foreign Minister Sergey Lavrov. "I urge Russia to play a constructive and cooperative role in the fight against ISIL. And to strive for a negotiated political solution to the conflict in Syria." 
 
 Conflicting reports on strikes in Syria 
 
 As diplomats talked, the violence in Syria continued unabated. 
 
 Syrian state media reported that joint Russian and Syrian airstrikes had hit ISIS targets in the modern city of Palmyra, adjacent to its ancient ruins. The airstrikes hit "ISIS hideouts," destroying 20 armored vehicles, three ammunition dumps as well as rocket launch pads, the SANA news agency reported, citing unnamed military sources. 
 
 The London-based Syrian Observatory for Human Rights reported that Russian strikes in and around Palmyra killed at least 15 ISIS militants and destroyed vehicles, posts and ammunition dumps there. Four more members of the terrorist group died in strikes in Raqqa, the Syrian city that has been referred to as ISIS' de facto capital. 
 
 But Russian state media denied the Palmyra strikes. 
 
 "All foreign media reports suggesting that Russian planes have allegedly launched airstrikes on the city of Palmyra are totally untrue," Defense Ministry spokesman Maj. Gen. Igor Konashenkov said, according to Tass 
 
 "We don't rule out that terrorists may be preparing provocations in this and other settlements — to blast mosques and then demonstrate fake photo and video materials with accusations against Russian aircraft on the territory of the Syrian Arab Republic," the general said in the Tass report. 
 
 However, another Russian state news agency, RIA Novosti, reported that Russian jets hit ISIS targets 28 kilometers (about 17 miles) from Palmyra. 
 
 Obama authorizes resupply of opposition 
 
 On Monday, U.S. President Barack Obama authorized the resupply of Syrian Kurds and the Arab-Syrian opposition as part of a strategy to put pressure on ISIS from the north and strengthen the border, according to two senior administration officials. 
 
 The approval came at a National Security Council meeting Thursday. 
 
 CNN reported over the weekend that Obama emphasized to his team that the United States would continue to support the Syrian opposition as Russia enters the war-torn country. 
 
 "President Obama was clear that we intend to continue our efforts to degrade and ultimately destroy ISIL in Syria and to keep supporting the moderate Syrian opposition," an official said. 
 
 JUST WATCHED Tony Blair: Russia seeking leverage from airstrikes Replay More Videos ... MUST WATCH Tony Blair: Russia seeking leverage from airstrikes 03:03 
 
 Russia's aims questioned 
 
 Russia has said it is conducting military operations to wipe out ISIS and other extremist groups. U.S. officials have seen the effort primarily as a move to bolster al-Assad, a close ally -- a goal Russian President Vladimir Putin does not deny. 
 
 It's believed the Russians are positioning weapons to support a Syrian ground offensive, officials said. 
 
 The equipment includes artillery as well as four BM-30 multiple-launch rocket systems -- all considered to be highly accurate. The latter is capable of rapid-fire rocket launches. 
 
 Officials: Russia brings electronic jamming equipment 
 
 Several weeks ago, Russia moved about half a dozen artillery pieces into the port of Latakia. 
 
 The United States originally thought the move might be for defense of the port, but the latest action is an indication of potential ground attacks in the coming days, an official said. 
 
 The weapons have been spotted between Homs and Idlib and west of Idlib. 
 
 It is not clear if they're now in final position for possible artillery strikes. 
 
 The officials also said Russia has moved electronic jamming equipment into Syria. Both a truck-mounted system and a number of pods that can go on aircraft have been observed. The equipment could potentially give the Russians the ability to jam electronics of U.S-backed coalition aircraft.
– Another game-changer? One of the United Nations' chief investigators says evidence is building that sarin gas was used in the Syrian conflict—but by rebels, not Bashar al-Assad's regime. "Our investigators have been in neighboring countries interviewing victims, doctors, and field hospitals" and "there are strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas," said Carla Del Ponte, a member of a panel probing alleged Syrian war crimes, Reuters reports. "This was use on the part of the opposition, the rebels, not by the government authorities," she adds. The war crimes inquiry commission Del Ponte is involved with is separate from the main UN probe into chemical weapons use in Syria. Each side accused the other of using chemical weapons during fighting in Aleppo in March, while Britain and France say they have evidence that regime forces used nerve agents in attacks on rebels. In other developments: Israeli airstrikes on targets inside Syria in recent days have killed at least 42 government soldiers, the Syrian Observatory for Human Rights tells the AP. Israel today emphasized that the strikes were not meant to aid rebels, Reuters adds, but rather to block weapons from Hezbollah. The Israeli move renewed debate on whether the US should use airstrikes against the Assad regime, the New York Times reports. John McCain, who yesterday blasted President Obama's "red line" as being "written in disappearing ink," said the strikes show that Syria's air defense system may not be much of an obstacle. Opposition activists say rebel forces shot down a military helicopter last night, killing eight government troops, the AP reports. The regime's air power has long been a chief target of rebels and activists say rebels also attacked and occupied parts of an air base in the north of the country yesterday, killing the base's commander and capturing a tank unit.