France has proof the Syrian government carried out a chemical weapons attack last week and would decide whether to strike back when all the necessary information had been gathered, President Emmanuel Macron said on Thursday.

"Our teams have been working on this all week and we will need to take decisions in due course, when we judge it most useful and effective," Macron told broadcaster TF1 when asked whether a red line had been crossed.

Macron said Paris had established that the government of Syrian President Bashar al-Assad had used chemical weapons, including chlorine, on April 7 in the town of Douma. He did not provide any details on the evidence or how it was acquired.

Watch: Shocking footage emerges as suspected gas attack kills at least 100

France is expected to join the United States and Britain in carrying out air strikes or some other form of attack in response for the chemical weapons' use, but it remains unclear when that might happen or even if it definitely will.

US President Donald Trump tweeted on Thursday morning: "Never said when an attack on Syria would take place. Could be very soon or not so soon at all!"

Macron said France wanted to remove the Syrian regime's chemical weapons capabilities. When asked whether those would be the targets of French strikes he said:

"When we decide it, and once we have verified all the information."

He said the priority was to avoid an escalation across the region.

"France will not allow any escalation that could harm the stability of the region as a whole, but we can't let regimes that think they can do everything they want, including the worst things that violate international law, to act," he said.