French President Emmanuel Macron said Thursday that there is evidence that the Syrian regime of Bashar Al Assad used chemical weapons.
"We have proof that... chemical weapons were used, at least chlorine, and that they were used by the regime of Bashar Al Assad," Macron said during an interview on France's TF1 television on Thursday.
Mr Macron said he would respond "at a time of our choosing, when we judge it to be the most useful and the most effective."
He indicated the response would come once all of the necessary information had been gathered and verifications carried out. Mr Macron did not supply the source of his information.
"France will not allow any escalation that could harm stability in the region… regimes that think they can do everything they want, including the worst things that violate international law, cannot be allowed to act," Mr Macron said.
France is poised to join punitive strikes on Syria over the alleged use of chemical weapons. The president set a "red line" on the use of such weapons last May.
US President Donald Trump is still weighing options for military action against Syria. Pentagon chief Jim Mattis and Mr Trump’s national security team met on Wednesday after the president warned Russia to expect a missile strike on Syria, tweeting missiles "will be coming." Mr Trump rowed back on that threat on Thursday, however, tweeting "Never said when an attack on Syria would take place. Could be very soon or not so soon at all!"
British Prime Minister Theresa May summoned her Cabinet back from vacation to discuss military action against Syria. Mrs May has indicated she wants Britain to join in any US-led strikes as the use of chemical weapons "cannot go unchallenged."
Ministers are expected to back Mrs May's call to join military action but some opposition politicians and Tories want the prime minister to allow MPs to vote on the plan before proceeding.
European Council on Foreign Relations senior policy fellow Julien Barnes-Dacey said it was difficult to see how the west could produce a wider strategy for Syria that allows for meaningful impact and limits escalation.
"I think the bellicose twitter language by President Trump and the forceful push back by the Russians has clearly left people somewhat rattled, and Europeans in particular want to dial things down before things get out of hand," Mr Barnes-Dacey told The National.
"More fundamentally, there does seem to be some awareness that another round of strikes is unlikely to effective without a wider strategy, which for the moment is sorely lacking. I suspect that the Americans and French have forced themselves into action and will still move militarily but the growing awareness of the difficulty of the situation is clearly injecting real caution into the mix," he added.
Kremlin spokesman Dmitry Peskov cautioned against "any steps which could lead to an escalation of tensions."
Moscow, which offers military support to the Syrian regime, has warned of a possible Russian response to a US attack.
It remains difficult to place an exact figure on the number of people killed and wounded Saturday in the rebel-held town of Douma, on the outskirts of Damascus.
The UN's World Health Organisation has requested access to verify reports that 70 people died, including 43 who showed "symptoms consistent with exposure to highly toxic chemicals."
The Syrian-American Medical Society, which operates in rebel-held areas, said more than 500 people had been treated for symptoms "indicative of exposure to a chemical agent."
A top Syrian rebel official told AFP on Thursday that his faction only agreed to abandon its battered enclave outside Damascus because of an alleged toxic gas attack.
"Of course, the chemical attack is what pushed us to agree" to a withdrawal from Douma, said high-ranking Jaish al-Islam member Yasser Dalwan.
It was the first public acknowledgement by Jaish al-Islam of a deal reached for Douma, their last rebel holdout in the Eastern Ghouta suburb of Damascus.
The agreement was announced on Sunday morning by Syria's government and its ally Russia, just hours after toxic gases were allegedly released on Douma.
First responders said more than 40 people died on Saturday after suffering symptoms consistent with chemical exposure, including wheezing, discoloured skin and foaming at the mouth.
Syria and its ally Russia have both denied the accusations. The global chemical watchdog said it would deploy a team to investigate.