President Barack Obama is preparing for the possibility of launching unilateral American military action against Syria.

Facing scepticism 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, Mr. 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,” said White House spokesman Josh Earnest.

Even before the vote in London, the U.S. was preparing to act without formal authorisation from the United Nations, where Russia has blocked efforts to seek a resolution authorising 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 on Thursday evening to explain why they believe Bashar Assad’s government was the culprit in a suspected chemical attack last week.

Legal rationale

Lawmakers from 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 Mr. Assad to the attack.

Afterward, the House Democratic leader, Rep. Nancy Pelosi pointedly sided with Republican Speaker John Boehner 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 Mr. Boehner and other lawmakers who say the administration needs to consult more with “all members of Congress” a reference to the limited circle briefed on Thursday night and provide “additional transparency into the decision-making process.”

Details

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 little new evidence backing up that conviction. It remained to be seen whether any sceptics 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 Dutch Ruppersberger, 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.

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