WASHINGTON — 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 U.S. 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 U.S. had expected Britain, a major ally, to join in the effort.

Top U.S. 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, to specify objectives and to lay out a firm case linking Assad to the attack.

Afterward, the House Democratic leader, Rep. Nancy Pelosi of California, pointedly sided with Republican Speaker John Boehner of Ohio in urging the administration to do more to engage with Congress on the matter, even as she expressed “my appreciation for the measured, targeted and limited approach the president may be considering.”

She said in a statement she agreed with Boehner and other lawmakers who say the administration needs to consult more with “all members of Congress” — a reference to the limited circle briefed Thursday night — and provide “additional transparency into the decision-making process.”

The high-level officials who spoke to the lawmakers offered more details of the suspected chemical attack and their firm conviction that the Syrian government was to blame but offered little new evidence backing up that conviction. 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 officials told lawmakers 1,300 men, women and children died in the attack, said Rep. C.A. Dutch Ruppersberger of Maryland, top Democrat on the House Intelligence Committee. That's a far higher death toll than has been reported; the humanitarian group Doctors Without Borders says the attack outside Damascus killed 355.

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 in 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.

“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.

Even so, 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.

An intelligence report similar to the findings shared with lawmakers Thursday night is expected to be released publicly today.

In London, Prime Minister David Cameron argued a military strike would be legal on humanitarian grounds, but he faced deep pressure from lawmakers and had already promised not to undertake military action until a U.N. chemical weapons team on the ground in Syria released its findings about the Aug. 21 attack.

United Nations experts are investigating the alleged use of chemical weapons in Syria as the United States and its allies prepare for the possibility of a punitive strike against President Bashar Assad’s regime, blamed by the Syrian opposition for the attack.The international aid group Doctors Without Borders says at least 355 people were killed in the Aug. 21 attack in a suburb of Damascus, the Syrian capital.Here’s a look at key Syria developments around the world Friday amid heightened tensions over potential military action:<B>FRANCE</B>French President Francois Hollande said his country can go ahead with plans to strike Syria for allegedly using chemical weapons despite the British Parliament’s failure to endorse military action. He told the newspaper Le Monde that the “chemical massacre of Damascus cannot and must not remain unpunished.”<B>SYRIA</B>U.N. experts began what is expected to be the last day of their investigation into the Aug. 21 attack. After an early morning delay, three U.N. vehicles left a Damascus hotel for more on-site visits. It was not immediately known where they were going.<B>RUSSIA</B>Presidential foreign policy adviser Yuri Ushakov expressed puzzlement over why the U.N. team was leaving so soon “when there are many questions about a possible use of chemical weapons in other areas in Syria.” He said Russia has not seen the U.S. intelligence that Washington claims proves the role of the Syrian government in last week’s alleged chemical weapons attack.<B>BRITAIN</B>Treasury chief George Osborne warned that Britain should not turn its back on the world after the stunning parliamentary defeat of a government motion for military intervention in Syria. He told the BBC there will be “national soul-searching” about Britain’s global role after the “no” vote.<B>GERMANY</B>German government spokesman Steffen Seibert said Germany isn’t considering joining military action against Syria and hasn’t been asked by others to do so. Berlin has called for the international community to take a “clear position” following the alleged chemical attack, but has left open what exactly that might entail.<B>IRAN</B>Followers of anti-American Shiite cleric Muqtada al-Sadr held rallies in Baghdad and the southern Iraqi city of Basra to denounce any western strikes against Syria. In the capital, about 2,000 Sadrists demonstrated while chanting anti-American slogans after prayers. About 3,000 Sadrists rallied in Basra, some carrying banners reading “No to America.”