Syrian forces loyal to President Bashar al-Assad carried out a "massive and co-ordinated" chemical attack on August 21, one of "at least three" this year, a declassified French intelligence report concluded yesterday.
The nine-page intelligence document, published on government websites, detailed five points suggesting pro-Assad fighters were behind the assault.
Satellite images showed strikes coming from government-controlled areas to the east and west of the Syrian capital and targeting rebel-held zones - areas that had since been bombed to wipe out evidence, the report concluded.
"The attack of August 21 can only have been carried out by the regime. Unlike previous attacks that used small amounts of chemicals and were aimed at terrorising people, this attack was tactical and aimed at regaining territory."
About 47 amateur video clips reportedly filmed on the morning of the attack and showing the impact on civilians had been authenticated by French military doctors, according to the intelligence. Video evidence alone showed that at least 281 people died in the attack but the real figure was more likely to be about 1500, it said.
Other French evidence gave details of suspected chemical attacks in the towns of Saraqib and Jobar in April which now appeared to have killed about 280 people, the report said.
"We believe the Syrian opposition does not have the capacity to carry out an operation of such magnitude with chemical agents," it concluded.
Speaking after presenting the files to French MPs, Jean-Marc Ayrault, the Prime Minister, said "nobody can deny" the reality of the August attack and that President Francois Hollande was "continuing efforts to forge a coalition as soon as possible" against Syria.
French MPs will debate the Syria crisis tomorrow, although Ayrault said there would be no vote. The President's aides have made it plain that France will not strike alone.
Assad challenged the West to provide "the slightest proof" that he used chemical weapons against his people.
He said any Western military intervention could lead to "regional war" and would harm "the interests of France".
"Whoever accuses must provide proof. We have challenged the United States and France to provide the slightest proof. Obama and Hollande have been incapable [of doing so] even to their own people," Assad told the French newspaper Le Figaro.
"Whoever contributes to financially or militarily bolstering terrorists is an enemy of the Syrian people," he said. "Whoever is against the interests of Syria and its people is an enemy."