"If the Americans, the French or the British had a shred of proof, they would have shown it beginning on the first day," he said, deriding Obama as "weak" and having buckled to U.S. domestic political pressure.
"We believe that a strong man is one who prevents war, not one who inflames it," Assad said.
French President Francois Hollande and Obama have been the two world leaders most vocally calling for action against Assad's regime, accusing it of carrying out a deadly chemical attack against rebel-held suburbs of Damascus on Aug. 21.
The U.S. said it has proof that the Assad regime is behind attacks that Washington claims killed at least 1,429 people, including more than 400 children. Those numbers are significantly higher than the death toll of 355 provided by the aid group Doctors Without Borders.
It has marked an intolerable escalation in a two-year civil war in Syria that has left some 100,000 people dead.
The Syrian government denies the allegations, and blames opposition fighters. In the Figaro interview, Assad questioned whether an attack took place at all and refused to say whether his forces have chemical weapons, as is widely believed.
If the U.S. and France strike, "Everyone will lose control of the situation ... Chaos and extremism will spread. The risk of a regional war exists," he added.
To back up its case, the French government published a nine-page intelligence synopsis Monday that concluded Assad's regime had launched an attack on Aug. 21 involving a "massive use of chemical agents," and could carry out similar strikes in the future.
In all, though, the French report provided little new concrete evidence beyond what U.S. officials provided over the weekend in Washington. Along with it, the French Defense Ministry posted on its Web site six clips of amateur video showing victims, some of which has already been widely available online and in the international media.
In the Figaro interview, Assad said "all the accusations are based on allegations of the terrorists and on arbitrary videos posted on the Internet."
The French report made no specific reference to the agencies involved or how the intelligence was collected about the attack, aside from referring to videos of the injured or killed, doctors' accounts, and "independent evaluations" such as one from Paris-based humanitarian aid group Doctors Without Borders three days after the attack.
A French government official, speaking on condition of anonymity because he wasn't authorized to speak about the matter because of its sensitivity, said the analysis was written by the spy agency DGSE and the military intelligence unit, DRM, and was based on satellite imagery, video images, and on-the-ground sources plus samples collected from the alleged chemical attacks in April.
The assessment said it was "very unlikely" that Syria's opposition had falsified images of suffering children that turned up online. It also said intelligence indicated the opposition "does not have the means to conduct such a large attack with chemical agents."
Around the time of the attack, Assad's regime feared a possible opposition strike on Damascus: "Our evaluation is that the regime was looking to loosen the vice and secure the strategic sites for the control of the capital," the report said.
The synopsis also said French intelligence services had collected urine, blood, soil and munitions samples from two attacks in April in Saraqeb and Jobar that confirmed the use of sarin gas.
France is "determined to take action against the use of chemical weapons by the regime of Bashar Assad, and to dissuade it from doing so again," Prime Minister Jean-Marc Ayrault said after hosting lawmakers to discuss the intelligence on Syria. "This act cannot go without a response."
France won't act alone and Hollande was "continuing his work of persuasion to bring together a coalition," Ayrault said. French parliament will debate the Syria issue Wednesday, but no vote is scheduled. The French constitution doesn't require such a vote for Hollande, though he could decide to call for one.
Russia, which along with Iran has been a staunch supporter of Assad through the conflict, brushed aside Western evidence of an alleged Syrian regime role.
"What our American, British and French partners showed us in the past and have showed just recently is absolutely unconvincing," Russian Foreign Minister Sergey Lavrov said Monday before the French report was released. "And when you ask for more detailed proof they say all of this is classified, so we cannot show this to you."
"There was nothing specific there, no geographic coordinates, no names, no proof that the tests were carried out by the professionals," he said, without identifying which tests.
Russian President Vladimir Putin proposed to send a delegation of Russian lawmakers to the U.S. to discuss the situation in Syria with members of Congress. Two top Russian legislators suggested that to Putin, pointing to polls that have shown little support among Americans for armed intervention in Syria.
On Sunday, U.S. Secretary of State John Kerry said Washington received new physical evidence in the form of blood and hair samples that show sarin gas was used in the attack. It wasn't immediately clear whether that evidence had been shared with Russia.
U.N. chemical inspectors toured the stricken areas last week, collecting biological and soil samples. A U.N. statement said the team "worked around the clock" to finalize preparations of the samples, which were shipped Monday afternoon from The Hague and would reach their designated laboratories "within hours," the statement said.
Secretary-General Ban Ki-moon planned to brief the Security Council's 10 non-permanent members on the Syria crisis Tuesday morning. Angela Kane, high representative for disarmament affairs, planned a Tuesday briefing for member states that requested the investigation of alleged chemical weapons use in the Ghouta area outside Damascus on Aug. 21.
The Obama administration has failed to bring together a broad international coalition in support of military action, having so far only secured the support of France.
Britain's Parliament narrowly voted against the country's participation in any military strike last week, despite appeals by Prime Minister David Cameron. The Arab League has stopped short of endorsing a Western strike against Syria.
In an emergency meeting Sunday, the 22-state League urged the United Nations and the international community to take "deterrent" measures under international law to stop the Syrian regime's crimes. Russia or China would likely veto any U.N. Security Council resolution sanctioning a Western strike against Syria.