France said on Tuesday it was certain the nerve agent sarin had been used in Syria on several occasions following tests it carried out on samples recovered there.
Increasing reports from the battlefield of the use of chemical weapons have lent urgency to a new diplomatic push to end the war and fuelled some calls for Western intervention in the conflict.
"These tests show the presence of sarin in various samples in our possession ... France is certain that sarin gas was used several times in Syria in limited areas," Foreign Minister Laurent Fabius said in a statement.
United Nations investigators said on Tuesday they had "reasonable grounds" to believe that limited amounts of chemical weapons had been used in Syria in a conflict where brutality was now a tactic of war.
France has been testing samples of suspected chemical weapon elements for several weeks, including some smuggled out by reporters from the French daily Le Monde.
"It would be unacceptable that those guilty of these crimes remain unpunished," Fabius said, without specifying whether Paris was able to tell who had used the gas.
The results had been handed to the Swedish head of a U.N. chemical weapons investigation team, Ake Sellstrom, he said.
Fabius said on Sunday said that if there was proof chemical weapons had been used there would be serious consequences, although Paris has not detailed what those would be.
A French diplomatic source said the samples had come from Jobar, just inside central Damascus, and Saraqib near the northern city of Idlib.
The sides in the conflict, now in its third year, have accused each other of using chemical weapons. President Bashar al-Assad's government has denied using chemical weapons and has in turn accused rebels of deploying them in the two-year civil war that the United Nations says has killed over 80,000 people.
UN investigators have been ready for weeks, but diplomatic wrangling and safety concerns have delayed their entry into Syria.
Syria, which is not a member of the anti-chemical weapons convention, is believed to have one of the world's last remaining stockpiles of undeclared chemical arms.