The US administration, also facing scepticism at home, shared intelligence with politicians aimed at convincing them the Syrian government used chemical weapons against its people and must be punished.
Despite roadblocks in forming an international coalition, President Obama appeared undeterred and advisers said he would be willing to retaliate against Syria on his own.
"The President of the United States is elected with the duty to protect the national security interests in the United States of America," White House spokesman Josh Earnest said.
Even before last night's British vote, the US was preparing to act without formal authorisation from the United Nations, where Russia has blocked efforts to seek a resolution sanctioning the use of force, or from Capitol Hill.
But the US had expected UK, a major ally, to join in the effort.
Top US officials spoke to selected politicians for more than 90 minutes in a late-night teleconference to explain why they believed Bashar Assad's government was the culprit in the suspected chemical attack last week.
Members of both parties have been pressing Mr Obama to provide a legal rationale for military action and specify objectives, as well as to lay out a firm case linking Assad to the attack.
Tennessee senator Bob Corker, the senior Republican on the Senate Foreign Relations Committee and already a supporter of moving against Syria in a limited way, said after the briefing that "strong evidence of the Assad regime's continued use of chemical warfare" merited a military response. He also participated in an unannounced, classified briefing given by the administration earlier yesterday, aides said.
It remained to be seen whether any sceptics were swayed by the teleconference, given the expectation that officials would hold back classified information to protect intelligence sources and methods.
"The main thing was that they have no doubt that Assad's forces used chemical weapons," New York congressman Eliot Engel, the top Democrat on the House of Representatives Foreign Affairs Committee and a supporter of Mr Obama's course, said after the briefing.
But he said the officials did not provide much new evidence of that.
"They said they have (intercepted) some discussions and some indications from a high-level official," he said, and that they possess intelligence showing material being moved in advance of the attack.
He called the tone "respectful", saying: "There was no shouting. No-one was accusing the administration of doing anything wrong."
Democratic senator Bob Menendez, chairman of the Foreign Relations Committee, said the briefing "reaffirmed for me that a decisive and consequential US response is justified and warranted to protect Syrians, as well as to send a global message that chemical weapons attacks in violation of international law will not stand."
In London, Prime Minister David Cameron argued a military strike would be legal on humanitarian grounds. But he faced deep pressure from MPs and had already promised not to undertake military action until a UN chemical weapons team on the ground in Syria released its findings about the August 21 attack.
In terse comments after last night's vote, he said that while he believed in a "tough response" to the use of chemical weapons, he would respect the will of the House of Commons.
Caitlin Hayden, Mr Obama's National Security Council spokeswoman, said the US would continue to consult with Britain but Mr Obama would make decisions based on "the best interests of the United States."
But it was not certain the US would have to act alone. France announced that its armed forces "have been put in position to respond" if President Francois Hollande committed forces to intervention against Syria. Mr Hollande does not need French parliamentary approval to launch military action that lasts less than four months.
Mr Obama discussed the situation in Syria with Republican House speaker John Boehner, who wrote to the president earlier this week seeking a legal justification for a military strike and the objectives of any potential action.
Assad, who has denied using chemical weapons, vowed his country "will defend itself against any aggression".
Some of the UN chemical weapons experts will travel directly from Syria on Saturday to different laboratories around Europe to deliver "an extensive amount of material" gathered, UN spokesman Farhan Haq said.
While the mandate of the UN team is to determine whether chemical agents were used in the attack, not who was responsible, Mr Haq suggested the evidence - which includes biological samples and witness interviews - might give an indication of who deployed gases.
Mr Obama and other top officials have not revealed definitive evidence to back claims that Assad used chemical weapons on Syrians. US officials say the intelligence assessments are no "slam dunk", with questions remaining about who actually controls some of Syria's chemical weapons stores and doubts about whether Assad himself ordered the strike.
Despite shortcomings in the intelligence, the White House signalled urgency in acting, with Mr Earnest saying the president believed there was a "compressed timeframe" for responding.
"It is important for the Assad regime and other totalitarian dictators around the world to understand that the international community will not tolerate the indiscriminate, widespread use of chemical weapons, particularly against women and children as they're sleeping in their beds," he said.
But many Congress members were pressing Obama to explain the need for military action and address fears that such a move might draw the U.S. deeper into the Syrian civil war.
Adam Smith, the senior Democrat on the House Armed Services Committee, warned that an attack might be ineffective and draw the United States into the Syrian civil war, now in its third year.
"Simply lashing out with military force under the banner of 'doing something' will not secure our interests in Syria," he said.
Republican Howard "Buck" McKeon, chairman of the House Armed Services Committee and a call participant, said administration officials were in the process of declassifying the evidence they had of the Syrian government using chemical weapons.
"When they do that, we'll understand. But it's up to the President of the United States to present his case, to sell this to the American public. They're very war weary. We've been at war now for over 10 years," Mr McKeon told reporters at a post-call news conference at his office in Valencia, California.
US defence secretary Chuck Hagel said the Obama administration was consulting allies to "further develop the facts" about the alleged chemical weapons attack in Syria and options for a response.
Speaking at a news conference in Manila in the Philippines, Mr Hagel said the administration would also continue to seek input from members of congress on how the US should respond to the deadly attack.
Mr Hagel said the consultation by top administration officials with congressional leaders was "not to convince anyone of anything". He said it was intended as an update and a chance to solicit politicians' views on possible US military or other action.
Asked what Assad could do now to avoid a US strike, Mr Hagel said: "I have not been informed of any change in the Assad regime's position on any issue."
On the British vote against military action in Syria, Mr Hagel said London had strongly and publicly condemned Syria's alleged gas attacks against civilians. "That vote in the parliament doesn't change that," he said.
He said Washington would keep talking to Britain and other nations on "ways forward together".