WASHINGTON—The Obama administration presented its case for possible military action against Syria to congressional leaders, including evidence from an intercepted phone call involving a high-level Syrian official, which a lawmaker said officials offered as proof of a government chemical-weapons attack. 
 
 The evidence was part of a 90-minute, unclassified briefing delivered to the congressional leaders in a conference call Thursday evening. 
 
 The administration officials made clear that its focus in Syria was on "preventing [President Bashar] al-Assad from using chemical weapons again," said a Democratic aide briefed on the call. 
 
 The administration wasn't deterred by Thursday's vote in the British parliament that prompted the government to drop its own military plans. 
 
 "The administration is figuring out the best way to do that and is seeking as much international support as possible, but won't let that dictate what our policy will be," the aide said. 
 
 Several lawmakers said they came away from the briefing with a conviction that the Assad regime was behind the attack, but that President Barack Obama must do more to persuade the country military action is needed and to rally international support. 
 
 Rep. Eliot Engel (D., N.Y.) said officials told the lawmakers in the call that they had intercepted a phone call from a high-level Syrian official discussing the attacks, which was proof the Assad regime was responsible for the chemical-weapons attack. 
 
 Mr. Engel, the senior Democrat on the House Foreign Affairs Committee, didn't say whether the officials elaborated on the evidence. 
 
 The Wall Street Journal reported Tuesday that the U.S. had such evidence, from Israeli spy services that obtained intelligence on a special Syrian unit overseeing Mr. Assad's chemical weapons. 
 
 Mr. Engel said in an interview that the administration officials made it clear the U.K. vote wouldn't determine what course of action the U.S. takes. 
 
 "The president will make a decision based on what is best for the U.S. and not what other countries would do," Mr. Engel said administration officials said on the call. 
 
 He said he agreed with this sentiment. "I don't think the U.K. ought to have a veto on what the U.S. does or doesn't do," he said. 
 
 In a statement after the briefing, Sen. Carl Levin (D., Mich.), the Senate Armed Services Committee chairman, called for a continuing effort to win international support for strikes against the Assad regime. 
 
 "I have previously called for the U.S. to work with our friends and allies to increase the military pressure on the Assad regime by providing lethal aid to vetted elements of the Syrian opposition," Mr. Levin said. "Tonight, I suggested that we should do so while UN inspectors complete their work and while we seek international support for limited, targeted strikes in response to the Assad regime's large-scale use of chemical weapons against the Syrian people." 
 
 Sen. Bob Corker (R., Tenn.), the senior Republican member of the Senate Foreign Relations Committee, said he opposed deploying American troops in Syria but would support "surgical, proportional military strikes, given the strong evidence of the Assad regime's continued use of chemical warfare." 
 
 He said he welcomed the consultation with key lawmakers in Congress, but said the administration would be "far better off if they seek authorization based upon our national interests, which would provide the kind of public debate and legitimacy that can only come from Congress." 
 
 Sen. Bob Menendez (D., N.J.), the chairman of the Senate Foreign Relations Committee, said in a statement that the briefing by the administration "reaffirmed for me that a decisive and consequential U.S. response is justified and warranted to protect Syrians, as well to send a global message that chemical-weapons attacks in violation of international law will not stand." 
 
 Congressional leaders on the call heard from Secretary of State John Kerry, National Security Advisor Susan Rice, Secretary of Defense Chuck Hagel, Director of National Intelligence James Clapper, and Navy Adm. James A. Winnefeld Jr., the vice chairman of the Joint Chiefs of Staff. 
 
 Unease among members of Congress about the prospect of U.S. involvement in a new military conflict in the Middle East continued to escalate on Thursday. 
 
 The numbers of lawmakers signing a letter to Mr. Obama calling on him to subject any potential military action to an authorization vote by Congress increased Thursday to 137 signatories, up from 116 on Wednesday. 
 
 Write to Corey Boles at corey.boles@dowjones.com and Janet Hook at janet.hook@wsj.com ||||| White House officials offered few details and no timeline for a possible attack on Syria during a 90-minute conference call on Thursday evening for members of Congress, according to a congressional source who participated in the call. 
 
 The call appears to have done little to change anybody’s mind. Members issued statements afterward reiterating their long-held views on Syria. 
 
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 Senate Armed Services Committee Chairman Carl Levin (D-Mich.) said the White House should seek international support for “limited, targeted strikes” against Syria, once again urging the administration to “increase the military pressure on the [Bashar] Assad regime by providing lethal aid to vetted elements of the Syrian opposition.” 
 
 (PHOTOS: Scenes from Syria) 
 
 Meanwhile, the committee’s ranking Republican, Sen. Jim Inhofe, was undeterred in his opposition to military operations in Syria until there’s a clear plan to pay for them. 
 
 “As I have said before, no red line should have even been drawn without first preparing a strategic plan and assessing our resources,” the Oklahoma Republican said following the unclassified briefing. 
 
 “The administration owes it to Congress and to the American people to lay out how they will fund their military action,” Inhofe said. “Is it going to be more furloughs?” 
 
 On the call, White House officials told lawmakers the president is still weighing his options on Syria, according to the congressional source. Several officials were on the call representing the administration, including Defense Secretary Chuck Hagel, Secretary of State John Kerry, White House National Security Adviser Susan Rice, Director of National Intelligence James Clapper and Vice Chairman of the Joint Chiefs of Staff Adm. Sandy Winnefeld. 
 
 (Also on POLITICO: Congress: Obama needs Syria OK from us) 
 
 In a statement, the White House said 15 members of Congress asked questions during the call and that it would continue reaching out to lawmakers. 
 
 “The views of Congress are important to the president’s decision-making process, and we will continue to engage with members as the president reaches a decision on the appropriate U.S. response to the Syrian government’s violation of international norms,” the White House said. 
 
 Members of Congress pressed the officials about a range of issues, the congressional source said, including how military operations in Syria would be funded. The officials offered no answer, the source noted, saying Hagel agreed it was an important question pledged to provide additional information before or soon after any attack was launched. 
 
 (PHOTOS: International response to Syria) 
 
 Even with such questions unanswered, however, advocates continued to push for military action. 
 
 Senate Foreign Relations Committee Chairman Robert Menendez (D-N.J.) said in a statement that “the use of chemical weapons by the Assad regime requires a decisive response.” 
 
 “This is not a moment to look the other way, to blind ourselves to the horrifying images in Syria, and to send the dangerous message to the global community that we would allow the use of a chemical weapons attack to take place with impunity,” Menendez said. 
 
 And Rep. Eliot Engel of New York, the ranking Democrat on the House Foreign Affairs Committee, said he agrees “with the president that the use of these weapons not only violates international norms, but is a national security threat to the United States.” 
 
 “The president’s national security team said that he is still weighing his options and will continue to consult with Congress,” Engel said. “The president’s team agrees that this type of action cannot go without consequences.” ||||| Story highlights The bill limits the authorization to 60 days, with an option for an additional 30 days 
 
 It makes clear there would be no U.S. armed forces on the ground 
 
 Kerry says Syria intelligence has been "scrubbed and re-scrubbed" 
 
 U.N. chief urges countries to "avoid further militarization of the conflict" in Syria 
 
 The Senate Foreign Relations Committee will take up a bipartisan authorization bill Wednesday for the use of force in Syria. 
 
 The resolution, which limits the authorization to 60 days with an option for an additional 30 days, was revised to address some of the concerns expressed during a Senate committee hearing Tuesday. 
 
 At the hearing, top U.S. officials faced tough questions from senators about plans for military strikes as House leaders lined up behind President Barack Obama's call to punish Syria for an August poison gas attack near Damascus. 
 
 Afterward, Foreign Relations Chairman Robert Menendez, D-New Jersey, said he and the committee's ranking Republican, Tennessee Sen. Bob Corker successfully negotiated the revised resolution. 
 
 According to a copy of that text, provided to CNN by a legislative source, the bill limits the authorization to 60 days with an option for an additional 30-day deadline. It also makes clear there would be no U.S. armed forces on the ground in Syria. 
 
 "Together we have pursued a course of action that gives the President the authority he needs to deploy force in response to the Assad regime's criminal use of chemical weapons against the Syrian people, while assuring that the authorization is narrow and focused, limited in time, and assures that the Armed Forces of the United States will not be deployed for combat operations in Syria," Menendez said in a statement. 
 
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 "With this agreement, we are one step closer to granting the President the authority to act in our national security interest," he added. 
 
 Kerry: Syria isn't Iraq, we've got the proof 
 
 Secretary of State John Kerry, Defense Secretary Chuck Hagel and Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, appeared before the Senate Foreign Relations Committee as the Obama administration lobbied Congress to authorize military action against Syria. From the outset, Kerry addressed the shadow of claims offered in the buildup to the 2003 invasion of Iraq, reassuring committee members that the intelligence linking Syrian government forces to the August 21 attack was solid. 
 
 "We are especially sensitive, Chuck and I, to never again asking any member of Congress to take a vote on faulty intelligence," said Kerry, who like Hagel, voted as a senator to authorize the Iraq invasion. "And that is why our intelligence community has scrubbed and re-scrubbed the evidence. We have declassified unprecedented amounts of information, and we ask the American people and the rest of the world to judge that information." 
 
 Kerry said that intelligence includes the trajectory of the rockets used to deliver the gas, Syrian military instructions to troops to don gas masks and concerns by top Syrian officials that U.N. inspectors would discover evidence of the attack. 
 
 "Not one rocket landed in regime-controlled territory -- not one," Kerry said. "All of them landed in opposition-controlled or contested territory." 
 
 Kerry said Obama isn't asking the United States to go to war -- just "to degrade and deter" the capacity of Syrian President Bashar al-Assad to launch another chemical attack. But he and Hagel faced tough questions from senators concerned that giving the administration the green light to attack Syria would draw the United States into that country's civil war, now 2 1/2 years old. 
 
 "Americans are understandably weary after the fiasco in Iraq and over a decade of war," said Sen. Tom Udall, D-New Mexico. "How can this administration make a guarantee that our military actions will be limited? How can we guarantee that one surgical strike will have any impact other than to tighten the vise grip that Assad has on his power, or allow rebels allied with al Qaeda to gain a stronger foothold in Syria?" 
 
 Kerry said the administration has no intention of sending American ground troops to Syria "with respect to the civil war." But he opposed any effort to put a ban on deploying ground forces into a congressional resolution authorizing military action, leaving open the possibility that U.S. troops may have to seize chemical weapons "in the event Syria imploded" or if extremist groups were poised to obtain them. 
 
 Hagel: Risks of action, risks of inaction 
 
 Hagel said any military action "would be limited in duration and scope," and won't be aimed at settling the Syrian civil war by force. He testified that the administration's proposal would not authorize strikes outside Syria, against any other government or against non-state groups. 
 
 "There are always risks in taking action, but there are also risks with inaction," he said. "The Assad regime, under increasing pressure by the Syrian opposition, could feel empowered to carry out even more devastating chemical weapons attacks." 
 
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 Likewise, Kerry warned that a failure to act would undermine American alliances, encourage other U.S. adversaries like Iran and North Korea and encourage al-Assad to use chemical weapons again. 
 
 U.S. officials have said the August 21 attack left more than 1,400 dead, and Obama said Tuesday that more than 400 of those were children. Syria denies government troops were behind the attack, arguing that they were the victims of a rebel chemical attack. 
 
 Syria's U.N. ambassador, Bashar al-Jaafari, told CNN's Christiane Amanpour that the allegations of poison gas use by his government "are false and unfounded." Al-Assad told the French newspaper Le Figaro on Monday that an attack on his country risks a regional war. 
 
 Administration officials and Obama himself have said the president has the authority to conduct military strikes even without congressional approval. But asked what the administration would do if Congress refused to authorize military action, Kerry said, "We're not contemplating that, because it's too dire." 
 
 Weapons inspectors from the United Nations are analyzing samples taken from the scene last week, but their results aren't expected to assign blame. The United States and its allies say the rebels have no capability to carry out a large-scale chemical weapons attack, however. 
 
 House leaders line up behind Obama 
 
 Earlier Tuesday, the leaders of both parties in the House of Representatives emerged from a White House meeting to support Obama's call for American strikes. 
 
 House Speaker John Boehner, R-Ohio, told reporters the use of poison gas was "a barbarous act" to which only the United States is capable of responding. House Minority Leader Nancy Pelosi, D-California, added that Washington must respond to actions "outside the circle of civilized human behavior." 
 
 But in a written statement later, Boehner said it is up to Obama "to make his case to the American people and their elected representatives" -- including securing support from individual members. 
 
 "All votes authorizing the use of military force are conscience votes for members, and passage will require direct, continuous engagement from the White House," the speaker said. 
 
 No vote is scheduled before Congress returns from its summer recess on Monday. Kerry defended the delay to seek congressional authorization, saying the move gives the United States time to make its case while adding pressure on al-Assad. 
 
 "This is working," Kerry said. "There are defections taking place. There's great uncertainty in Syria. We are building support, a greater understanding, and I would far rather be playing our hand than his at this point in time." 
 
 Most of the focus of administration lobbying has been on the House. In the Senate, a Democratic source familiar with Majority Leader Harry Reid's thinking told CNN that Reid is confident any authorization measure will pass his chamber. The source said it is likely 60 votes will be needed to overcome a filibuster, and Reid thinks the votes are there. 
 
 U.N. chief calls for end to Security Council 'stalemate' 
 
 The United Nations has said more than 100,000 people -- including many civilians -- have been killed in Syria since a popular uprising spiraled into a civil war in 2011. Syrian opposition activists reported another 77 dead on Tuesday, mostly in Damascus and its suburbs. 
 
 New U.N. figures point to the staggering impact the war has had on the nation. 
 
 The number of Syrians who have fled their war-ravaged country has risen above 2 million, the U.N. refugee agency reported, an increase of nearly 1.8 million people over the past 12 months. 
 
 But at the United Nations, Secretary-General Ban Ki-moon urged Security Council members to await test results on the samples collected by U.N. inspectors. 
 
 "We should avoid further militarization of the conflict and revitalize the search for a political settlement," Ban said. "I take note of the argument for action to prevent further uses of chemical weapons. At the same time, we must consider the impact of any punitive measure on efforts to prevent further bloodshed and facilitate a political resolution of the conflict. The turmoil in Syria and across the region serves nobody." 
 
 He urged member states to work through the Security Council, where Syrian allies Russia and China are expected to block any call for military action. 
 
 "The Security Council has a duty to move beyond the current stalemate and show leadership," Ban said. "This is a larger issue than the conflict in Syria. This is about our collective responsibility to humankind." 
 
 And al-Jaafari, the Syrian envoy, told CNN that all Syrians will be victims "of any escalation of the situation." 
 
 "We don't need wars. We need peaceful settlement of conflicts according to the charter of the United Nations," he said. 
 
 During Tuesday's hearing, Udall questioned whether the United States should be doing more to pressure Russia and China rather than going around the Security Council. Kerry said the Russians refused to sign off even on a resolution condemning a chemical attack that didn't assign blame. 
 
 "That doesn't mean we should turn our backs and say there's nothing we can do," Udall said. 
 
 But Kerry responded that if the Security Council is being blocked, "That doesn't mean we should turn our backs and say there's nothing we can do." 
 
 U.N. inspectors await test results 
 
 U.N. weapons inspectors left Syria on Saturday with samples that will help determine whether chemical weapons were used in the August 21 attack. Those samples will all be at laboratories by Wednesday and will be tested "strictly according to internationally recognized standards," Ban told reporters at U.N. headquarters. 
 
 Those tests could take up to three weeks -- and even then, those tests will only determine whether a gas attack took place, not who was behind it. 
 
 Kerry said Sunday that hat blood and hair samples taken from medics at the scene of the alleged attack point to the nerve agent sarin. The samples reached U.S. hands "through an appropriate chain of custody, from east Damascus, from first responders," he said. 
 
 But David Kay, the former head of the U.S. weapons hunt in Iraq, said the "shadowing effect" of that war makes the U.N. inspectors' jobs more difficult. 
 
 "Remember, these results will be analyzed and re-analyzed around the world," said Kay, the former U.N. weapons inspector who ultimately determined Iraq had dismantled the chemical, nuclear and biological weapons programs used to justify the American-led invasion in 2003. "So as an inspector, you want to get it right rather than necessarily get it quick" 
 
 Kay told CNN the question has been settled for the Obama administration "and for many Americans, including myself. But that's not enough, because of Iraq."
– The specifics of President Obama's plans for Syria don't seem much clearer after a conference call with Congressional leaders. The call didn't include information on when a strike would occur, Politico notes, nor did it appear to sway lawmakers who had doubts about military action. During the call, which involved top officials including John Kerry, Chuck Hagel, and Susan Rice, the administration discussed an intercepted phone call it called proof the Syrian government used chemical weapons, the Wall Street Journal reports—though the call doesn't directly implicate Bashar al-Assad, the New York Times notes. The White House's goal is "preventing Assad from using chemical weapons again," a Democratic aide said. As to the British vote against using force, the administration said that "the president will make a decision based on what is best for the US and not what other countries would do," said Rep. Eliot Engel, the top Democrat on the House Foreign Affairs Committee. "I don't think the UK ought to have a veto on what the US does or doesn't do." Officials said Obama still hadn't settled on a course of action—but the Times says "all indications" suggest a strike on the heels of the UN's departure from Damascus tomorrow. But we may know more soon: A senior administration official tells CNN the administration will today release declassified intelligence supporting the claim that the Syrian regime launched a chemical attack.