White House Principal Deputy Press Secretary Josh Earnest speaks about Syria during a press briefing at the White House in Washington August 29, 2013. [Photo/Agencies]

WASHINGTON - US President Barack Obama on Thursday prepared for the possibility of launching unilateral American military action against Syria within days as Britain opted out in a stunning vote by Parliament.

Facing skepticism at home, too, the administration shared intelligence with lawmakers aimed at convincing them the Syrian government used chemical weapons against its people and must be punished.

Despite roadblocks in forming an international coalition, 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 the vote in London, the US was preparing to act without formal authorization from the United Nations, where Russia has blocked efforts to seek a resolution authorizing the use of force, or from Capitol Hill. But the US had expected Britain, a major ally, to join in the effort.

Top US officials spoke with certain lawmakers for more than 90 minutes in a teleconference Thursday evening to explain why they believe Bashar Assad's government was the culprit in a suspected chemical attack last week. Lawmakers from both parties have been pressing 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.

A number of lawmakers raised questions in the briefing about how the administration would finance a military operation as the Pentagon is grappling with automatic spending cuts and reduced budgets.

Sen. Jim Inhofe of Oklahoma, the top Republican on the Armed Services Committee and a participant on the call, said in a statement that the administration presented a "broad range of options" for dealing with Syria but failed to offer a single plan, timeline, strategy or explanation of how it would pay for any military operation.

It remained to be seen whether any skeptics were swayed by the call, given the expectation in advance 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 Rep. Eliot Engel, top Democrat on the House Foreign Affairs Committee and a supporter of Obama's course, said after the briefing.

But he said the officials did not provide much new evidence of that.

Democratic Sen. 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."