He and Senator Lindsey Graham, a fellow Republican foreign policy hawk, emerged from the Oval Office meeting on Monday cautiously optimistic that Mr Obama would step up support for the rebels.

"There seems to be emerging from this administration a pretty solid plan to upgrade the opposition," Mr Graham said.

He added that he hoped the opposition would be given "a chance to speak directly to the American people" to counter US fears that they were dominated by al-Qaeda sympathisers.

"They're not trying to replace one dictator, Assad, who has been brutal... to only have al-Qaeda run Syria," Mr Graham said.

The US announced in June, following the first allegations the Assad regime had used chemical weapons, that it would send light arms to the rebels but refused to provide anti-aircraft missiles and other heavy weapons.

American concerns were born partly out of the experience of Afghanistan in the 1980s, when CIA weapons given to the anti-Russian mujahideen were later used by the Taliban.