A French intelligence report says forces loyal to Bashar al-Assad carried out a "massive" chemical attack last month.
Anxious to overcome widespread scepticism of military action in response to the alleged gas attack, France released a nine-page report outlining its case against the regime.
Based on military and foreign intelligence services, the report says the regime launched an attack "combining conventional means with the massive use of chemical agents" on rebel-held areas around the capital Damascus on August 21.
It says that based on videos, French intelligence has counted at least 281 dead but that reports of up to 1500 killed were consistent with such heavy use of chemical weapons.
"The attack on August 21 could only have been ordered and carried out by the regime," 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 said.
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A government source said separately that imagery obtained by French intelligence shows "the launch zone for the rockets was held by the regime".
Washington and Paris are pushing for military action but are facing deep public scepticism, despite the deaths of more than 110,000 people since an uprising against Syria's regime began in March 2011.
French Prime Minister Jean-Marc Ayrault presented the intelligence report to top MPs, ahead of a parliamentary debate on the Syria crisis on Wednesday.
The French president can order military action without parliamentary approval but some politicians have urged President Francois Hollande to put the issue to a vote, as President Barack Obama is doing in the US.
Obama was to continue calls on Monday to lobby members of Congress to back military action, after a surprise move at the weekend putting off threatened missile strikes.
In a rare interview with Western media released on Monday, Assad warned that Western military strikes risked setting off a wider conflict in the Middle East.
"We cannot only talk about a Syrian response, but what could happen after the first strike," Assad said.
"Everyone will lose control of the situation once the powder keg explodes. Chaos and extremism will spread. There is a risk of regional war."
He also said France should consider the consequences of taking part in military action.
"There will be repercussions, negative ones of course, on the interests of France," Assad said.
France has emerged as the main US ally in the Syria crisis after the British parliament, in a shock move, rejected plans for military action mooted by Washington.
