A report presented to parliament by Prime Minister Jean-Marc Ayrault says the assault on 21 August involved the "massive use of chemical agents".
Syrian President Bashar al-Assad has again denied carrying out the attack, telling the French newspaper Le Figaro it would have been "illogical".
He also warned that foreign military action could ignite the "powder keg" of the wider region.
Gavin Hewitt reports. Watch BBC video » | Monday, September 02, 2013
