France's President Emmanuel Macron says he has "proof" that the Syrian government attacked the town of Douma with chemical weapons last weekend, BBC reported on Thursday.
He said he would decide "in due course" whether to respond with air strikes.
Western states are thought to be preparing for missile strikes in response to the alleged attack.
The French leader had previously said any strikes would target the Syrian government's "chemical capabilities".
He did not give the source of his information but said: "We have proof that last week chemical weapons, at least chlorine, were used by the regime of Bashar al-Assad."
Asked in a TV interview whether France would join strikes on Syria, he said: "We will need to take decisions in due course, when we judge it most useful and effective.
"France will not allow any escalation that could harm stability in the region," he said. But, he added: "Regimes that think they can do everything they want, including the worst things that violate international law, cannot be allowed to act."
Activists and medics say dozens of people died when government aircraft dropped bombs filled with toxic chemicals on Douma on Saturday.
But President Assad's government denies being behind any chemical attack.
The international Organisation for the Prohibition of Chemical Weapons (OPCW) has said it will go to Douma "shortly", but it is unclear when it would arrive and how much evidence of any chemical attack might remain.
Douma was the last opposition stronghold near Damascus. Local activists say the main leaders of the group that held it have left, following an agreement between Russia and the rebels.
Russian claims that the town had been taken over by Syrian forces could not be independently verified.