Forces loyal to Bashar al-Assad carried out a "massive" chemical attack last month, a French intelligence report said, as the Syrian leader warned military strikes against him could would ignite a regional war.
Anxious to overcome widespread scepticism of military action in response to the alleged gas attack, France on Monday 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 21 August.
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 21 August could only have been ordered and carried out by the regime," the report said.
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.
US President Barack Obama shocked Washington and the world on Saturday when he decided to seek support for military action in Syria from the US Congress, when it seemed US cruise missile strikes on Assad's forces and assets were imminent.
Hawkish senators John McCain and Lindsey Graham appeared to offer qualified backing for Obama's plans.
"A vote against that resolution by Congress I think would be catastrophic," said McCain.
"It would undermine the credibility of the United States of America and the president of the United States. None of us want that," he added.
Obama has stressed that any US action, expected to include cruise missile attacks, would be "limited" and "narrow."
But Assad, in a rare interview with Western media released Monday, 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 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 rejected involvement in any military action.
Britain's government said Monday it had "no plans" to hold a second parliamentary vote on the issue.
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 Obama is doing in the United States.
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.
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.
US Secretary of State John Kerry and Defense Secretary Chuck Hagel will testify to the Senate Foreign Relations committee Tuesday in what will be one of the most high profile political set pieces in Washington in weeks.
America's top military officer, General Martin Dempsey, chairman of the joint chiefs of staff, will also appear.
Kerry "will argue that the failure to take action against Assad unravels the deterrent impact of the international norm against chemical weapons use; endangers our friends and our partners along Syria's borders... and risks emboldening Assad and his key allies - Hezbollah and Iran," a senior State Department official said on condition of anonymity.
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.
Meanwhile Italy warned that the European Union lacks an adequate approach for the refugee problem that will worsen in case of an international intervention in Syria.