President Barack Obama and his top aides launched a full-scale political offensive on Sunday, September 1 to persuade a skeptical Congress to approve a military strike against Syria, but faced a struggle to win over lawmakers from both parties and a war-weary American public, Reuters said.

Obama made calls to members of the House of Representatives and Senate, with more scheduled for Monday, underscoring the task confronting the administration before it can go ahead with using force in response to a deadly chemical attack blamed on the Syrian government.

Dozens of lawmakers cut short their vacations and streamed into the corridors of the Capitol building for a Sunday afternoon intelligence briefing on Syria with Obama's national security team.

When they emerged nearly three hours later, there was no immediate sign that the many skeptics in Congress had changed their minds. Many questioned the broad nature of the measure Obama is seeking, suggesting it needed to be narrowed.

Most seemed convinced that Syria had engaged in chemical warfare. But the credibility of the administration's intelligence is turning out to be a less important issue than the nature and usefulness of the response.

Lawmakers questioned the effectiveness of limited strikes, the possible unintended consequence of dragging the United States into another open-ended Middle East conflict, the wisdom of acting without broader international backing to share the burden, and the war fatigue of the American public.

Polls show the public is largely opposed to U.S. military action.

Obama is taking a gamble by putting the brakes on the military assault he considers essential to maintain U.S. credibility after he had said the use of chemical weapons would constitute a "red line" for the United States.