A French intelligence report said Monday that forces loyal to Bashar al-Assad carried out a "massive" chemical attack last month, as the Syrian leader warned that military strikes against him would risk igniting a regional war.
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 said 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 said that based on videos, 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 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.
A government source said separately that imagery obtained by French intelligence showed that "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 lawmakers, ahead of a parliamentary debate on the Syria crisis on Wednesday.
The French president can order military action without parliamentary approval but some lawmakers have urged President Francois Hollande to put the issue to a vote, as President Barack Obama is doing in the United States.
Obama was to continue calls Monday to lobby members of Congress to back military action, after a surprise move at the weekend putting off threatened missile strikes.
That delayed any US-led military action until at least September 9, when US lawmakers return from their summer break.
With the administration keen to keep the pressure on, Secretary of State John Kerry and Defence Secretary Chuck Hagel will testify to the Senate Foreign Relations committee on Tuesday.
US Senator John McCain meanwhile said that a failure by Congress to authorise military action in Syria would be "catastrophic" because it would undermine US credibility.
In a rare interview with Western media released Monday, Assad warned that Western military strikes risked setting off a wider conflict in the Middle East.
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"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 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.
Britain's government said Monday it had "no plans" to hold a second parliamentary vote on joining military action.
Ayrault said there would be no vote during Wednesday's French parliamentary debate and that Hollande was "continuing efforts to forge a coalition as soon as possible" to take action.
"There is no question of France acting on its own," he added.
It was unclear who else may be persuaded to take part in Western action, but NATO chief Anders Fogh Rasmussen said Monday he was personally convinced the chemical attack had taken place and that Assad's regime was responsible.
He called for a response that would "send a very clear message" against the use of chemical weapons, but said any military action should be "very short, sharp (and) tailored."
Efforts to win UN backing for military action have been stymied by Russia, which said Monday it remained totally unconvinced that the regime carried out the attack.
Foreign Minister Sergei Lavrov warned that Western military action would sink joint efforts by Washington and Moscow to organise a peace conference in Geneva.
"If the action announced by the US president -- to the great regret of all of us -- does in fact take place... it will put off the chances of (holding) this conference for a long time, if not forever," Lavrov said.
China, which in the past has joined Russia in blocking UN Security Council resolutions against Assad, also said it was "gravely concerned" about the prospect of "unilateral military actions" against Syria.
Fighting continued to rage in Syria meanwhile, with the Syrian Observatory for Human Rights watchdog saying that nearly 90 rebels were killed near Damascus in the previous 48 hours.