Speaking to news channel CNEWS, he added: "These crimes must not go unpunished. In any case, there are investigations, United Nations commissions... there will be a war crimes trial."
At least 86 people were killed on Tuesday in rebel-held Khan Sheikhun in northern Syria in a suspected chemical attack that left people choking and foaming at the mouth.
France is again pushing for a resolution at the United Nations to condemn the attack blamed by the West on Assad's regime, but Ayrault did not sound optimistic after the first discussions on Wednesday at the international body.
"It's difficult because up to now every time we have presented a resolution, there has been a veto by Russia and sometimes by China ... but we must cooperate because we need to stop this massacre," he added.
Moscow, which launched a military intervention in 2015 in support of Assad's forces, has defended the Syrian government against accusations it is responsible for the attack.
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