France has proof that the Syrian government launched chlorine gas attacks, President Emmanuel Macron said.
Mr Macron said France would not tolerate “regimes that think everything is permitted”.
Speaking on TF1 television, Mr Macron said “we have proof that chemical weapons were used, at least chlorine” in recent days by Syrian President Bashar Assad’s government.
He did not say whether France is planning military action against Assad’s government.
Mr Macron said he has been talking regularly this week with US President Donald Trump about the most effective response.
With increasing concerns about a US-Russia proxy war in Syria, Mr Macron insisted that “France will not allow an escalation or something that could damage the stability” of the region.
On Tuesday, Mr Macron said any French action would target Syria’s chemical weapons abilities.
Syrian opposition activists and medics say a suspected gas attack last week in Douma killed more than 40 people.
The Syrian government has denied the allegations.
Chancellor Angela Merkel said Germany will not participate in possible military action in Syria, but supports sending a message that the use of chemical weapons is unacceptable.
She said she spoke on Thursday with Mr Macron.
Mrs Merkel said in Berlin: “Germany will not take part in possible military action – I want to make clear again that there are no decisions – but we see, and support this, that everything is being done to send a signal that this use of chemical weapons is not acceptable.”