France says tests on samples from two sites prove that sarin gas has been used in the conflict in Syria.
Experts say one of the cases can be traced back to the Syrian government.
Foreign Minister Laurent Fabius says a line has been crossed and all options, including military action, are on the table.
“On the one hand, we have been able to analyse samples gathered in the field by journalists from Le Monde,” he said in a televised interview, “and on the other we have been able to follow the chain of events back to its origins and the lab’s conclusion is clear: it is sarin gas.”
“The other question is can we trace the people responsible? There is no doubt at all in the second case – it is the regime and its supporters.”
The news comes the same day as the UN published a report describing Syria as “in free-fall”.
French sources hope the results of the tests will help UN investigators push their case to enter the country. Teams have so far been held up by diplomatic wrangling and safety concerns.
Bashar al-Assad’s government has denied using chemical weapons during the course of the two-year conflict and has in turn accused rebels of deploying them.