Media caption Victoria Nuland, US state department, says they fear the regime may be planning an attack on al-Haffa 
 
 The US has expressed fears that the Syrian government "may be organising another massacre" in the town of Haffa in Latakia province, where UN military observers have been denied access. 
 
 UN Secretary General Ban Ki-moon said it was vital that the observers should be allowed into the town. 
 
 He and peace envoy Kofi Annan expressed grave concern about violence there. 
 
 A BBC correspondent travelling with UN observers witnessed sustained and heavy shelling in the old city of Homs. 
 
 The UN observers are in Syria to monitor the implementation of a putative ceasefire, brokered by Mr Annan, which has all but collapsed. 
 
 'Civilians trapped' 
 
 Annan's six-point plan 1. Syrian-led political process to address the aspirations and concerns of the Syrian people 2. End to violence by all sides; army troops to stop using heavy weapons and withdraw to barracks 3. Parties to allow humanitarian aid 4. Authorities to free political detainees 5. Authorities to ensure freedom of movement for journalists 6. Authorities to allow peaceful demonstrations 
 
 "The UN made a particular appeal to be able to get into Haffa because it had heard concerning reports about regime plans," US state department spokeswoman Victoria Nuland told a news briefing in Washington. 
 
 "At the same time, if you follow Syrian media, they were claiming that the opposition would cause a massacre." 
 
 Earlier this month, activists said Syrian government forces killed 108 people in the region of Houla, in Homs province, and 78 people in the village of Qubair, in Hama province. 
 
 But Ms Nuland downplayed the idea of foreign intervention in Syria, saying the emphasis would remain on using political and economic measures to pressure the government of President Bashar al-Assad. 
 
 The Syrian government blames the violence on foreign-backed armed terrorist gangs. 
 
 In a statement, Mr Annan, joint envoy for the UN and Arab League, said there were "indications that a large number of civilians are trapped" in Homs and Haffa. 
 
 Later Mr Ban spoke of the "great danger" to civilians under fire. 
 
 "The secretary general underlines the importance of unimpeded access by UNSMIS to Haffa, amid reports of a build-up of government forces around the town," he said in a statement. 
 
 Syrian government helicopter gunships strafed rebel positions in Haffa, UN observers and human rights activists said. 
 
 Media caption The BBC's Paul Danahar: "I was watching a mortar land pretty much every minute" 
 
 Heavy fighting was also reported in Rastan and Talbisa, north of Homs city, "with artillery and mortar shelling, as well as firing from helicopters, machine guns and smaller arms", said a statement from the UN Supervision Mission in Syria (Unsmis). 
 
 A car bomb exploded in the city of Deir al-Zour, killing 10 people, the London-based Syrian Observatory for Human Rights said, quoting sources on the ground. 
 
 Fighting was also reported in Idlib province. 
 
 It is impossible to confirm these reports independently, as Syria heavily restricts journalists' freedom of movement. 
 
 Russia, which supports the Syrian government, called for Iran to take a role in ending the 15-month old conflict. 
 
 Last week, US Secretary of State Hillary Clinton rejected the idea of involving Iran, which she accused of "helping to stage-manage the repression" in Syria. 
 
 Russian Foreign Minister Sergei Lavrov will travel to Tehran on Wednesday to discuss the initiative, the foreign ministry said. ||||| (CNN) -- Government military forces fired indiscriminately Monday from helicopters on a town on the outskirts of Jabal Al-Zawiya, inflicting scores of casualties on civilians and rebel forces, an activist told CNN. 
 
 Ibrahim Swed, speaking from the town in northwestern Syria's Idlib province, said that the fighting between rebel forces in the Free Syrian Army and government forces continued for six hours and resulted in 32 deaths and more than 50 wounded. 
 
 Opposition activists said at least 93 people were killed across the country Monday, 35 of them in Idlib. 
 
 "The regime is escalating the use of violent forces," said an activist in Idlib whom CNN is identifying only as Ahmad for safety reasons. "We cannot believe that the world is watching us being killed ... we want military intervention." 
 
 Ahmad said more than 40 Syrians were killed Monday in Idlib province. 
 
 Battle for largest cities may have begun 
 
 In Washington, State Department spokeswoman Victoria Nuland said the attacks showed the government to be desperate. 
 
 The use of helicopters to fire on civilians and the use of pro-regime thugs called Shabiha "constitutes a very serious escalation," Nuland told reporters. "What government voluntarily uses helicopters and fires from them on their own civilians if they're not desperate?" she asked. "What government depends on a bunch of thugs in trucks, irregulars, if they're not desperate? So, you know, clearly the government is under threat." 
 
 British Foreign Secretary William Hague said Monday that "all options" should be considered if the peace plan put forth by special envoy Kofi Annan fails to achieve peace. 
 
 "On 1st April, the Syrian regime committed itself to implementing the Annan Plan and on 12th April announced a ceasefire," Hague said in a statement. "They have not kept either of these commitments." 
 
 Hague added that the plan "cannot be used indefinitely by the regime to play for time. If the Annan plan is not implemented, we will argue for a new and robust U.N. Security Council resolution aimed at compelling the regime to meet its commitments under the plan, and requiring all parties to comply with it." 
 
 He continued, "We do not want to see the Annan plan fail, but if despite our best efforts it does not succeed, we would have to consider other options for resolving the crisis, and in our view all options should then be on the table." 
 
 Hague's remarks came as Annan, the U.N.-Arab League joint special envoy, said he was "gravely concerned" about reports that fighting between Syrian government and opposition forces had escalated. 
 
 "He is particularly worried about the recent shelling in Homs as well as reports of the use of mortars, helicopters and tanks in the town of Al-Haffa, Lattakia," Annan spokesman Ahmad Fawzi said Monday in a statement. "There are indications that a large number of civilians are trapped in these towns." 
 
 The statement called for the protection of civilians and demanded that U.N. observers be granted immediate access to Al-Haffa. 
 
 Nuland said U.S. officials joined Annan in expressing alarm over reports "that the regime may be organizing another massacre, this time in the village of Al-Haffah in Latakia province, as well as in the towns of Dar Azure (ph), in Daraa, in Homs, in Hama and in the suburbs of Damascus." 
 
 She said U.N. military observers had been trying to reach Al-Haffah but had been blocked by government forces. 
 
 In a note to correspondents from the spokeswoman for the U.N. Supervision Mission in Syria (UNSMIS), Sausan Ghosheh said U.N. observers reported heavy fighting in Rastan and Talbiseh, north of Homs, with artillery and mortar shelling and firing from helicopters, machine guns and smaller arms. 
 
 Heavy artillery shelling and machine gun firing were heard and seen over the Homs neighborhood of Khaldiyeh, she said. 
 
 "UNSMIS has also received reports of a large number of civilians, including women and children trapped inside the town and are trying to mediate their evacuation," she said. 
 
 An online video purporting to show live images of Homs on Monday showed more than a dozen explosions in an hour. 
 
 Shells rained on Deir Ezzor province, where eight bodies were found after regime forces raided a city at dawn, the opposition Local Coordination Committees of Syria said. 
 
 In southwestern Syria, explosions and gunfire targeted the city of Douma as forces conducted raids and made arrests, the LCC said. 
 
 The state-run Syrian Arab News Agency reported that 25 members of the army and law enforcement as well as one civilian who had been "targeted by armed terrorist groups while they were in (the) line of duty" were buried Monday. 
 
 Over the weekend, the opposition Syrian National Council elected minority Kurdish activist Abdul Basit Sieda to unite dissidents seeking to oust Syrian President Bashar al-Assad. 
 
 Al-Assad defined by indecision, violence 
 
 Sieda, a native of Hasaka, Syria, who lives in Sweden, called on officials in Syria, Russia and China "to think carefully about the situation now because the whole stability of the region -- if not the whole stability of the world -- is at stake here. We would like to call upon them to support the Syrian people." 
 
 Russia and China have blocked U.N. Security Council resolutions that many other nations said could have pushed al-Assad to stop the killing. The two countries, which have major trade ties with Syria, said they want more balanced resolutions that call for a cessation of violence on all sides. 
 
 Russia is viewed as a key ally of Syria. While Western countries have criticized Russia for its arms trade with Syria, Russia has insisted it is not propping up al-Assad's regime. 
 
 Russian Foreign Minister Sergey Lavrov said Saturday there was no alternative to Annan's peace plan, despite evidence that it's being violated daily. 
 
 "The situation looks more and more grim," Lavrov said. "For the first time since the beginning of this crisis, we see the question of foreign intervention. And our position remains unchanged. We will never agree to sanction the use of force in the U.N. Security Council." 
 
 The United Nations has said at least 9,000 people have died since the crisis erupted in March 2011. Opposition groups, however, say the toll is higher, with estimates ranging from at least 12,000 to more than 14,000. 
 
 CNN cannot independently confirm reports of casualties or violence in Syria, as the government has restricted access by international journalists. 
 
 Dozens of countries have recognized the Syrian National Council as a legitimate representative of the Syrian opposition, though many members of the group's leadership are expatriates. 
 
 Asked how he planned to bridge a gap between the Syrian opposition in exile and the opposition inside the country, Sieda said, "We are in direct communication and contact with revolutionary forces inside. We are always communicating with them. ... The relationship between us and the forces inside has never been stronger." 
 
 But al-Assad has said he will not deal with opposition members influenced from the outside. 
 
 While Sieda railed against the violence committed by al-Assad's forces, the regime itself announced a new stage in its argument that "armed terrorist groups" -- and not government forces -- are responsible for the violence. 
 
 A program on state-run TV said "that terrorists of various nationalities from the terrorist organization Jabhet al-Nasra, which is affiliated with al Qaeda, planned and carried out" bombings in Damascus on March 8. The cars used in the attack were driven by a Jordanian terrorist and a Syrian Palestinian, "and were trailed by an Iraqi," the state-run news agency SANA reported. 
 
 CNN's Hamdi Alkhshali, Holly Yan, Josh Levs, Salma Abdelaziz, Saad Abedine and Ivan Watson contributed to this report. ||||| Media playback is unsupported on your device Media caption UN mission head Maj-Gen Mood: "Deaths were an unacceptable attack" Amateur footage contained within this clip purportedly shows a mass burial 
 
 UN observers have counted at least 90 bodies, including 32 children, after a Syrian government attack on a town. 
 
 UN mission head Maj-Gen Robert Mood told the BBC the killing in Houla was "indiscriminate and unforgivable". 
 
 UK Foreign Secretary William Hague said he would seek a strong global response to the "appalling crime". UN chief Ban Ki-moon said it was a "flagrant violation of international law". 
 
 Syria's government has blamed the deaths on "armed terrorist gangs". 
 
 This is one of the bloodiest attacks in one area since the uprising against President Bashar al-Assad began in March 2011. 
 
 Activists say some of the victims died by shelling, while others were summarily executed, or butchered by the regime militia known as the "shabiha". 
 
 UN-Arab League peace envoy Kofi Annan, and the Arab League have also condemned Friday's assault. 
 
 France's Foreign Minister Laurent Fabius said he was making immediate arrangements for a Paris meeting of the Friends of Syria group, which includes Western and Arab nations, but not Russia or China, who have blocked previous attempts to introduce UN sanctions. 
 
 Fighting in Syria has continued despite the deployment of some 250 UN observers monitoring a ceasefire brokered by Mr Annan - a ceasefire which the BBC's Jim Muir in neighbouring Lebanon says is now "pretty fictional". 
 
 The UN says at least 10,000 have been killed since the protests began. 
 
 UN 'refused to come' 
 
 A joint statement from Mr Ban and Mr Annan said firing heavy weapons into a residential neighbourhood was a "flagrant violation of international law" which must stop and those responsible should be held to account. 
 
 Analysis Initial reports from activists of more than 90 civilians killed in a "massacre" at Houla were met with the usual caution. But a visit by the UN monitors confirmed that the reality was as horrendous as the gruesome internet videos portrayed. The observers have been criticised for not preventing the carnage in the first place. But they were at least able to verify what happened, and document the deadly use of government artillery and tank fire, even if they could not explain the circumstances leading to the carnage. That means that if [UN-Arab League envoy] Kofi Annan goes to Damascus next week, as he is expected to do in a bid to salvage his six-point peace plan, his argument that government troops and tanks must withdraw will be strengthened. But with the Free Syrian Army threatening to scrap even notional commitment to the battered ceasefire, events on the ground are clearly outpacing peace efforts. With international action from the Security Council ruled out, the trend towards arming the rebels and fuelling a civil war may gather force. 
 
 Mr Annan would be contacting Syria's government to "convey in the clearest terms the expectations of the international community, and he will also do so during his forthcoming visit to Syria", it said. 
 
 Mr Hague said he would be calling for an urgent session of the UN Security Council in the coming days. 
 
 Arab League head Nabil al-Arabi called the assault a "horrific crime" and urged the Security Council to "stop the escalation of killing and violence by armed gangs and government military forces," the Reuters news agency reports. 
 
 The opposition Free Syrian Army says it can no longer commit to the ceasefire unless the Security Council can ensure that civilians are protected, the AFP news agency reports. 
 
 Horrific video footage has emerged from Houla of dozens of dead children, covered in blood, their arms and legs strewn over one another. It is unverified, but our correspondent says such images would be difficult to fake. 
 
 International media cannot report freely in Syria and it is impossible to verify reports of violence. 
 
 A team of UN observers visited the town on Saturday and afterwards Maj-Gen Mood said they could confirm "the use of small arms, machine gun[s], artillery and tanks." 
 
 But he did not say who was behind the killings. 
 
 Our correspondent says local people are angry that the observers failed to intervene to stop the killing. 
 
 Abu Emad, speaking from Houla, said their appeals to the monitors failed to produce action. 
 
 "We told them at night, we called seven of them. We told them the massacre is being committed right now at Houla by the mercenaries of this regime and they just refused to come and stop the massacre." 
 
 The opposition Syrian National Council (SNC) said more than 110 people died. The SNC's Ausama Monajed told the BBC the regime was selecting vulnerable towns to "teach the entire country a lesson". 
 
 "It is beyond humanity what we have seen," he said. 
 
 Media playback is unsupported on your device Media caption UK Foreign Office Minister Alistair Burt says the massacre was "an act of pure, naked savagery" 
 
 Activists called a day of mourning on Saturday. 
 
 Earlier, in a letter to the Security Council, Mr Ban said the Syrian opposition controlled "significant parts of some cities". 
 
 He said that "established terrorist groups" could have been behind some of the recent bomb blasts in Syria judging from the sophistication of the attacks. 
 
 He said the situation remained "extremely serious" and urged states not to arm either side in the conflict. 
 
 Earlier this month, a bombing in Damascus left 55 dead in an attack which the government blamed on al-Qaeda. The attack came amid mounting fears that the terrorist group was taking advantage of the conflict to gain a foothold. 
 
 On Thursday, a UN-mandated panel said Syrian security forces were to blame for most abuses in the conflict, which has continued despite the presence of the UN observers. 
 
 Mr Annan's six-point peace agreement ordered a cessation of violence on 12 April. While casualties appeared to fall after the truce, the fighting quickly resumed to previous levels.
– The US and UN are warning that the Syrian government may be organizing another massacre, this time in the town of Haffa, where UN monitors have been impeded, reports the BBC. International envoy Kofi Annan says he is "gravely concerned" about the escalation of violence, and reports of mortar, tank, and helicopter attacks on Haffa and its surrounding villages. "A large number of civilians are trapped in these towns," he warned. The Syrian regime is using "new horrific tactics" on rebellious areas, a State Department spokeswoman says, warning: "People will be held accountable." A new UN reports has, for the first time, placed the Syrian government and its allied militias on a list of groups that kill and sexually attack children, AP reports. The report found that in Syria, children as young as nine have been the victims of killing, torture, sexual violence, and arbitrary detention, and have been used as human shields.