US Secretary of State John Kerry said Thursday he had seen "raw data" that suggests that chlorine has been used in the Syrian conflict, supporting accusations made by France against the regime.

"I have seen evidence ... it's not verified yet... hasn't been confirmed," he told reporters after a meeting of the Friends of Syria group in London.

"But I've seen the raw data that suggests that there may have been, as France has suggested, a number of instances in which chlorine has been used in the conduct of war.

"If it has, and if it could be proven, then that would be against the agreements of the chemical weapons treaty, against the weapons convention that Syria has signed up to."

French Foreign Minister Laurent Fabius said on Tuesday that the regime is believed to have used chemical weapons, including chlorine, in 14 attacks since October 2013.

Syria signed the Chemical Weapons Convention last year as part of a deal to surrender its chemical weapons arsenal after it was accused of a sarin attack in the suburbs of Damascus.

Possessing chlorine is not a violation of the convention, but the treaty prevents the use of the gas as a weapon.

Human Rights Watch, the New York-based watchdog, also said this week that the evidence "strongly suggests" Syria's government used chlorine gas on three towns in mid-April.

The Organisation for the Prohibition of Chemical Weapons watchdog said last month it would look into the attacks and has dispatched a team to carry out investigations on the ground in Syria.