"There is no doubting the responsibility of the Syrian regime given the way that the sarin [gas] used was produced," French Foreign Affairs Minister Jean-Marc Ayrault told journalists after a secret service report was presented to a meeting of French defence chiefs on Wednesday.
Tests made on unexploded ordnance and on victims found a chemical fingerprint is "typical of the method developed in Syrian laboratories", he said, adding that the production method "bears the regime's hallmarks and allows us to determine its responsibility for this attack".
He said the substance France believes was used in the attack, in which 31 children were among the 87 dead, contains hexamine, a component that was also found in a gas attack in Saraqeb in north-west Syria in 2013.
Assad, backed by his ally Russia, denies that Syrian forces are using chemical weapons and claims to have handed over stockpiles under a Russian-brokered deal in 2013.
National Front demands proof
The vice-president of France's National Front, Florian Philippot, demanded proof of Ayrault's claim on Wednesday, recalling the US government's allegations that Saddam Hussein had chemical weapons before the 2003 invasion of Iraq.
He called for a UN inquiry, "not an inquiry by M Ayrault".
After the attack US President Donald Trump ordered air strikes on a Syrian airbase, claiming it was launched from there.
On Monday the US government placed 271 chemists from the Syrian Scientific Studies and Research Centre (SSRC) and other officials on its financial blacklist in response to their presumed role in the chemical weapons attack.