PARIS (Reuters) - France said on Thursday samples it had collected suggesting Syrian government forces had used chlorine gas in the country’s civil war may not prove to be conclusive and would need to be cross-checked with other information to determine its use.

Women, affected by what activists say was a gas attack, receive treatment inside a makeshift hospital in Kfar Zeita village in the central province of Hama May 22, 2014. REUTERS/Badi Khlif

The Organization for the Prevention of Chemical Weapons (OPCW) has opened an investigation into the alleged chlorine attacks, more than a dozen of which have been reported since April 11 in several areas.

France, one of Assad’s fiercest critics, was the first Western power to provide non-lethal military aid to rebels. It has been a vocal critic of United States policy on Syria since President Barack Obama backed down from launching air strikes following suspected chemical attacks last year.

Syria agreed to hand over its entire chemical weapons stockpile after hundreds were killed in a sarin gas attack near Damascus. But Assad has denied using chemical weapons.

In a daily briefing to reporters, French foreign ministry spokesman Romain Nadal said Paris, which has been examining about 14 samples for several weeks, would continue to do so in coordination with other nations.

“Given that ... chlorine which is widely used for civilian purposes is very volatile, the results of the analysis may not necessarily prove to be conclusive, (and) will need to be complemented with other information,” Nadal said.

A French diplomatic source said among complementary information being studied was communication between Syrian officials, pieces of debris suspected to have been used in launching chlorine gas and medical evidence from people reported to have been affected by a gas.

“If there is no smoking gun, you need to put all the pieces of the puzzle together,” said the source, adding that Paris was working with its main allies United States and Britain on building an exact picture of what happened.

Another alleged chlorine attack took place on May 22 the day that Russia and China vetoed a French-drafted resolution to refer the situation in Syria to the International Criminal Court for possible prosecution of war crimes and crimes against humanity.

Chlorine is likely to be less lethal than sarin but its use as a weapon is illegal under a global chemical weapons convention signed by Syria.

Its use would also breach the terms of a deal last year between Washington and Moscow, itself now weeks behind schedule, aimed at ridding Syria of its chemical arsenal.

Syria did not declare chlorine as part of its stockpile, further complicating the action to rid Assad of chemical arms.

“We must increase our pressure on the Damascus regime and its backers to put a definitive end to the chemical threat that it places on its population,” Nadal said.