Forces loyal to Syrian President Bashar al-Assad carried out a "massive and coordinated" chemical attack on August 21, according to a declassified French intelligence report.
The nine-page document, released on Monday, lays out five points that suggest Assad was behind the attacks in suburbs of the capital, Damascus.
The AFP news agency quoted the report as saying: "The Syrian regime launched an attack on some suburbs of Damascus held by units of the opposition, combining conventional means with the massive use of chemical agents."
"We believe the Syrian opposition does not have the capacity to carry out an operation of such magnitude with chemical agents," it said.
It said that based on video reports, French intelligence had counted at least 281 dead, but that reports of up to 1,500 killed were consistent with such heavy use of chemical weapons. The US estimates are 1,429 dead, including more than 400 children.
"The attack on August 21 could only have been ordered and carried out by the regime," the report said, adding that the affected areas were then heavily shelled to destroy evidence of the chemical attack.
At a news briefing, Jean Marc Ayrault, the French prime minister, said: "On the 21st of August the regime of Assad used massive chemical weapons for the oppression of his people."
"No one denies the reality. Elements we have obtained allow us to hold the regime responsible. This act cannot remain unanswered.
"What is at stake is to discourage all from using such weapons again. To dissuade Assad from using them again by applying firm action.
"Our aim is not to overthrow the regime. There will only be a political solution in Syria. It is not for France to act alone. France respects international law."
The report was released as the French newspaper, Le Figaro, published excerpts of an interview with Assad, in which he said that the Middle East was a "powder keg" and that any intervention would make it explode.
'Powder keg' warning
Assad challenged Western powers including the US and France to show the world proof that his government was behind the chemical attack.
Assad said: "Anyone who accuses must provide evidence. We challenged the United States and France to show us proof."
In-depth coverage of escalating violence across Syria
He said any attack on Syria would bring the Middle East to conflagration:
"The Middle East is a powder keg, and the fire is approaching today. We must not only talk about the Syrian response, but what might happen after the first strike. Nobody knows what will happen," Assad said.
"Everyone will lose control of the situation when the powder keg explodes. Chaos and extremism will spread. The risk of a regional war exists."
Earlier, Sergey Lavrov, Russia's foreign minister, rejected Western intelligence as absolutely unconvincing."
He said "there was nothing specific there, no geographic coordinates, no names, no proof that the tests were carried out by the professionals". He did not describe the tests further, although his US counterpart, John Kerry, says the US has field evidence which shows sarin gas was used in the August 21 attack.
"What our American, British and French partners showed us in the past and have showed just recently is absolutely unconvincing," Lavrov said.
"And when you ask for more detailed proof they say all of this is classified so we cannot show this to you."