US Secretary of State John Kerry said Wednesday he had asked his French counterpart Laurent Fabius to share evidence of Syria's apparent use of chemical weapons against rebel forces.

"I asked him... whether he could send us the information that shows us the chain of custody of that evidence, so we know precisely where it came from," Kerry said on the sidelines of an Organization of American States meeting.

White House spokesman Jay Carney did not give a definitive response Tuesday to the French claim, but said Washington was working with France, Britain and the Syrian opposition to probe the possible use of chemical weapons.

On Wednesday, Kerry said Washington was completing its own analysis and that he was "very comfortable" with the US government's "own calendar" on how and when to act.

"Make no mistake whatsoever: the president's red line is real. He has a set of options which are alive and waiting," Kerry said.