President Bashar al-Assad warned Syria would retaliate if France takes part in foreign strikes on his forces, while Paris said it had intelligence proving Assad had ordered chemical attacks and was determined to punish him.
"If the policies of the French state are hostile to the Syrian people, the state will be their enemy," Assad said in an interview with French newspaper Le Figaro. "There will be repercussions, negative ones obviously, on French interests."
France has backed Syrian rebels since the start of the two-and-a-half year old civil war. It fears the violence may spill into Lebanon, where about 20,000 French citizens live, many French companies operate and where France has an 800-strong contingent of U.N. peacekeepers.
President Francois Hollande, along with U.S. President Barack Obama, has said Assad should be punished for an attack on August 21 in which Washington says more than 1,400 people were killed, many of them children. The Syrian government says it was carried out by rebels.
Assad said it would have made no sense to use chemical weapons in an area where his troops were also fighting.
"Those who make accusations must show evidence. We have challenged the United States and France to come up with a single piece of proof. Obama and Hollande have been incapable of doing so," said Assad, according to extracts of the interview published on Monday.
The prospect of an imminent Western strike on Syria has receded since Britain's parliament last week voted against taking part in any action against Syria and Obama decided to seek the approval of Congress before any assault.
France, whose president has the constitutional right to make war, remains a forceful advocate of Western military action.
On Monday, senior members of Hollande's ruling party rebuffed opposition calls for a parliamentary vote to approve any attack.
After presenting an intelligence report on the chemical attack to lawmakers on Monday, French Prime Minister Jean-Marc Ayrault appeared as determined as ever to use force against Assad, saying Paris wanted to prevent him from using such weapons again and dissuade others from copying him.
"This act cannot be left without a response," Ayrault said. "It's not for France to act alone. The president is continuing his work of persuasion to bring together a coalition.
"The objective is neither to topple the regime nor liberate the country." he said.
The nine-page intelligence document drawn up by France's military and foreign intelligence services and made public late on Monday listed points suggesting Assad's fighters were behind the "massive and coordinated" chemical attack on August 21.
It said the opposition had neither the know-how nor the experience to carry it out.
"This (the attack) poses a major threat to national and global security," a French official told Reuters ahead of the report's release.
Satellite imagery showed strikes came from government-controlled areas to the east and west of the Syrian capital and targeting rebel-held zones, just before civilians began dying from gas poisoning, the official said.
Later, the neighborhoods where the strikes were launched were bombed and fires lit purposefully to wipe out evidence, said the official.
"Unlike previous attacks that used small amounts of chemicals and were aimed at terrorizing people, this attack was tactical and aimed at regaining territory," the official said.
According to the intelligence report, aerial and artillery conventional bombing took place between 3 a.m. and 4 a.m. in the Damascus suburb of East Gouta.
In parallel, the areas of Zamalka, Kafr Batna and Ayn Tarma were hit with chemical attacks and at 6 a.m. a ground offensive was launched by the regime in those areas, the report said.
"Our services have information ... that suggests other acts of this nature could be carried out," it said.
In the interview in Le Figaro, Assad said the Middle East was a powder keg and a Western attack on Syria could push the entire region into chaos.
"We shouldn't just talk about a Syrian response, but what will happen after the first strike," Assad said. "Everybody will lose control of the situation when the powder keg blows. There is a risk of a regional war."