France aims to build a coalition of countries to back military action against the Syrian government in response to a chemical weapons attack in Damascus, Prime Minister Jean-Marc Ayrault said on Monday.

"This act cannot be left without a response," Ayrault said after presenting an intelligence report on Syria to lawmakers. "It's not for France to act alone. The president is continuing his work of persuasion to bring together a coalition without delay."

"France is determined to penalize the use of chemical weapons by [President Bashar] Assad's regime and to dissuade with a forceful and firm response," Ayrault said. "The objective is neither to topple the regime or liberate the country." he said, adding that only a political solution in Syria was possible.

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Assad earlier Monday scorned allegations that his forces were behind chemical attacks in Damascus last month and warned that any French military action against his government would trigger negative repercussions.

Assad's remarks came after a French government source said that a declassified report had revealed that the Syrian president carried out a "massive and coordinated" chemical attack on August 21.

In an interview with the French daily Le Figaro, the Syrian president rebuffed both U.S. and French declarations that they had proof of his government's involvement in the attack.

"Those who make accusations must show evidence. We have challenged the United States and France to come up with a single piece of proof. [Presidents Barack] Obama and [Francois] Hollande have been incapable of doing so," Assad told Le Figaro.

"Anybody who contributes to the financial and military reinforcement of terrorists is the enemy of the Syrian people. If the policies of the French state are hostile to the Syrian people, the state will be their enemy," he said. "There will be repercussions, negative ones obviously, on French interests."

The nine-page French document - issued by external and military intelligence services and to be presented to lawmakers later on Monday - lays out five points that suggest Assad was behind the attacks. "This poses a major threat to national and global security," the source said.

The dossier is central to Hollande's calls for Assad to be punished with military action for the reported chemical attack on areas controlled by Syrian rebels during their 2-1/2-year-old uprising.

Britain's parliament last week voted against taking part in any action against Syria and U.S. President Barack Obama, who also says he thinks Assad's forces were to blame, has decided to seek Congress approval before any assault.

"This [the attack] poses a major threat to national and global security," the French source said .

Satellite imagery showed strikes coming from government-controlled areas to the east and west of the Syrian capital and targeting rebel-held zones - areas that have since been bombed to wipe out evidence, the source added.

"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 source said.

Around 47 amateur video clips reportedly filmed on the morning of the attack and showing the impact on civilians had been authenticated by French military doctors, according to the intelligence.

Other French evidence gave details of other suspected chemical attacks, in the towns of Saraqib and Jobar in April, which now appeared to have killed about 280 people, the source said.

Other sources have previously told Reuters that the blood and urine samples used for the intelligence were taken after a Syrian government helicopter dropped munitions on April 29 at Saraqib, near the northern city of Idlib.

France had also been testing for sarin in samples of suspected chemical weapons smuggled out by reporters from the French daily Le Monde at Jobar, near central Damascus, in mid-April.

The source said the intelligence had confirmed those attacks took place.

U.S. prepares for potential strike

Meanwhile, a U.S. administration official said the White House is prepared to rework language in a draft resolution authorizing military force in Syria to address concerns from lawmakers.

The official said the administration was open to changes "within the parameters that [the] president has previously explained."  

Earlier Monday, the USS Nimitz aircraft carrier and four other ships in its strike group moved into the Red Sea in what U.S. defense officials described as "prudent planning" in case the vessels were needed for military action against Syria.

U.S. officials said that the strike group sent to the Red Sea had not received any orders to move into the Mediterranean, where five U.S. destroyers and an amphibious ship remain poised for possible cruise missile strikes against Syria.

Along with these developments, NATO Secretary-General Anders Fogh Rasmussen said on Monday he had seen evidence convincing him that Syrian authorities were behind a deadly chemical weapons attack and said it would send a "dangerous signal to dictators" if the world did not react with a "firm response."

Hackers attack Marines site

Also on Monday, computer hackers aligned with Assad struck an Internet recruiting site for the U.S. Marine Corps, urging troops to "refuse your orders" if the United States attacks Syria.

The attack appeared to be the work of the Syrian Electronic Army, which also recently targeted the New York Times' website and Twitter.

The hackers' message to the Marines said the Syrian Army "should be your ally, not your enemy" against "a vile common enemy" of terrorism. "Refuse your orders," said the message, which included six photos of people in military-style uniforms, their faces obscured and holding hand-written messages, such as "I will not fight for Al-Qaida in Syria."

A Defense Department spokesman said the site, on commercial network rather than the Defense Department network, had been restored after an outage of a few hours.