Barack Obama is preparing for the possibility of unilateral American military action against Syria within days, hours after Britain opted out in a stunning Commons vote.

Meanwhile 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.

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