All the available evidence points to a "massive and co-ordinated" chemical weapons attack by the Syrian government against its own people, one that could soon be repeated, according to a French intelligence memo released Monday as part of President François Hollande's effort to build support for military intervention.

The scale and lethality of the assault with sarin gas, as well as the military tactics used to deploy it, point the finger squarely at the regime of Syrian President Bashar al-Assad, the document says.

"Our intelligence services have information from Syrian sources that suggest there could be further actions of a similar nature," says the nine-page compilation, put together by France's main foreign-intelligence agency, the DGSE, and its military intelligence.

For his part, Assad said in an interview published Monday that U.S. President Barack Obama and Hollande "have been incapable" of proving the Syrian government was behind the Aug. 21 attack.

The French spy services released the summary report as Hollande emerged as the most vocal international leader calling for armed reprisals against the Assad regime for its alleged use of the nerve agent sarin on rebel fighters.

Britain, Germany and Canada have rejected an armed assault on Syrian government forces, while Obama, initially hawkish in calling for a "timely" and "firm" response, now says he will take the debate to Congress next week.

France's spy agencies released this nine-page synopsis report on Monday making the case that the regime of Syrian President Bashar al-Assad was behind a deadly chemical attack on August 21. (Thomas Coex/AFP/Getty)

Hollande has said France will not intervene alone in the 2½-year-old Syrian civil war, which has so far resulted in 100,000 deaths and the displacement of a third of the country's population. That leaves the French president struggling to build a coalition of partners.

The declassified dossier his agencies released Monday states in forceful terms that Syria "has one of the biggest active stockpiles of chemical weapons, part of a long-standing and diversified program that has been monitored for some time by France's intelligence services and those of its main allies."

It says Syria has three main chemical agents in its stockpile — sarin, mustard gas and the most toxic military nerve agent known, VX — as well as rockets capable of launching them up to 500 kilometres.

Those weapons are all maintained by a military unit composed entirely of Alawite Muslims noted for the fierce loyalty to Assad, himself an Alawite, the report says.

And it says the order to deploy any chemical agents must come from the top, from Assad himself or a select number of high-ranking officials of his regime.

The Aug. 21 attack was "clearly" their doing, the document concludes.

It used a "classic strategy" of artillery bombardments by the Syrian army, followed by a ground assault, that incorporated the gas deployment as part of a "coherent tactic." The spy agencies of France's allies noted a Syrian army buildup and preparations in the days before, the report affirms.

"It is clear, from studying the attack sites, that only the regime could have assaulted strategic opposition positions in this way."

As for the Assad regime's claims that rebel fighters launched the gas, the dossier concludes that "the Syrian opposition doesn't have the capability" for the large-scale co-ordinated manoeuvre.

"Manipulation by the opposition is highly improbable," the document says.

Sarin used

While the alleged chemical weapons attack on Aug. 21 was by far the biggest of the Syrian civil war, resulting in an estimated 281 to 1,500 deaths, it was not the first.

Journalists from the French newspaper Le Monde who spent two months reporting on the civil war in the Damascus area said they witnessed the Assad regime deploy sarin on a smaller scale against rebels in mid-April.

The newly released French intelligence dossier confirms that incident, and says another attack two weeks later in Saraqeb, a city in northwestern Syria, also had strong evidence of chemical weapons.

In that case, French agents obtained blood, urine, soil and munitions samples from victims and attack sites.

"Analysis confirmed the use of sarin," the dossier says.

'Absolutely unconvincing'

Evidence collected by Western countries has so far done little to sway Syria's major allies. Russian Foreign Minister Sergey Lavrov said Monday that information presented by the United States to Moscow of the Assad regime's alleged chemical weapons use was "absolutely unconvincing. There was nothing specific there, no geographic co-ordinates, no names, no proof that the tests were carried out by the professionals."

Sen. John McCain, a Republican from Arizona, left, accompanied outside the White House by Sen. Lindsey Graham, a Republican from South Carolina, tells reporters U.S. President Barack Obama will have a high threshold to meet to convince Congress an attack on Syria is warranted. (Evan Vucci/Associated Press)

Russian President Vladimir Putin had already said on the weekend that the idea of a military intervention in Syria would be "foolish nonsense" that "defies all logic."

In Washington, Obama sought to sow support among legislators for a strike on Syria by first persuading two Republican senators known as prominent foreign-policy hawks.

Both Senator John McCain of Arizona, Obama's one-time rival for the presidency, and Senator Lindsey Graham of South Carolina said Obama must make a strong case for attacking Syria if he wants to win congressional backing for the operation.

McCain told reporters at the White House that a military intervention now will be more difficult because Assad "is moving his forces around."

Both McCain and Graham questioned the wisdom of the administration publicly signalling in advance its intention to strike.

The Republican senators, who often speak with the same voice on foreign affairs, talked in the White House driveway Monday after a private meeting with Obama.