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. 
 
 Text Size - 
 
 + 
 
 reset 
 
 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.”
– 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.