"If the red line has been crossed, there will be a response," he told Europe 1 radio, adding that intelligence shared by President Emmanuel Macron and US President Donald Trump "in theory confirms the use of chemical weapons."
In a phone call Monday night, the two leaders again discussed the latest alleged chemical attack in Syria's civil war, in the city of Douma near Damascus on Saturday.
Rescuers and medics in Douma say more than 40 people died after the suspected poison gas attack in the last rebel-held pocket of the one-time opposition stronghold of Eastern Ghouta.
France has repeatedly warned that evidence of the use of chemical weapons in Syria is a "red line" that would prompt French strikes on Syrian government forces.
Trump for his part said Monday that "we have a lot of options militarily and we'll be letting you know pretty soon... probably after the fact."
In April last year, Trump launched a cruise missile strike against a Syrian air base after a previous chemical weapons attack UN monitors later pinned on Assad's regime.
The UN Security Council, gathered in New York, is expected to vote as Tuesday on rival US and Russian proposals to probe chemical attacks in Syria.
Moscow has denounced the claims as "fabrications", with its UN envoy warning Monday that the possibility of military action was "very, very dangerous".