Turkey has said the results of post-mortem examinations show that chemical weapons were used in an attack which killed at least 70 people in Syria's Idlib province.
Thirty-two victims of Tuesday's attack were brought to Turkey and three have subsequently died.
US President Donald Trump accused Syrian President Bashar al-Assad's government of going "beyond a red line" and said his attitude toward Syria and Assad Mr had changed.
However, France has cautioned against rushing towards a military solution and said it is still seeking a United Nations Security Council resolution on Syria.
Foreign Minister Jean-Marc Ayrault said that diplomatic negotiations were a priority over military action in Syria.
Asked whether France could join any possible US military action on Syria, Mr Ayrault said: "The first stage is to get a resolution vote and above all to re-start peace negotiations in Geneva.
"It is not to go in ourselves, under the pretext that the US President may have a rush of blood to the head, and get onto a war footing".
Syria's armed forces "did not and will not" use chemical weapons, even against jihadist groups, Foreign Minister Walid Muallem said.
"I stress to you once again: the Syrian army has not, did not, and will not use this kind of weapons - not just against our own people, but even against the terrorists that attack our civilians with their mortar rounds," he said.