UN chemical weapons investigators will not explicitly pin the blame on anyone in their upcoming report on the 21 August gas attack in Syria, but diplomats say their factual reporting alone could suggest which side in the country's civil war was responsible.

The report could easily become a bargaining chip in talks between Moscow and Western powers on conditions for Syria to give up its chemical weapons and the terms of a UN Security Council resolution on the matter.

Two Western diplomats said they strongly expected chief UN investigator Ake Sellstrom's report would confirm the US view that sarin gas was used in the attack on suburbs of Damascus that killed hundreds.

French Foreign Minister Laurent Fabius said Thursday that UN inspectors would "probably" publish their report Monday.

Fabius said there was little doubt that Al-Assad's forces, which he said possessed 1,000 metric tons of chemical weapons, were behind the attack.

Diplomatic sources said earlier that the UN report may come out Monday, as early as this weekend, or later next week.

While Sellstrom's report will not explicitly assign blame, Western diplomats said UN Secretary-General Ban Ki-moon, who has been highly critical of Syrian President Bashar Al-Assad's government during the 2.5 year civil war, may choose to say whether or not he feels the facts suggest Al-Assad's forces were responsible.

"We expect it (the report) will have a narrative of evidence," said one UN official. A third Western diplomat said the report will not directly accuse anyone of carrying out the attack, but it may include facts that suggest blame.

Two Western diplomats following the issue said they expected those facts would indirectly point in the direction of the Syrian government. They declined to elaborate.

Foreign Policy's blog "The Cable" cited diplomats Thursday voicing similar views — that the facts in Sellstrom's report would suggest the Al-Assad government's culpability.

Such facts could include the trajectories of the projectiles loaded with gas, indicating whether they came from government or rebel-held areas. It could also involve looking at the areas that were attacked, the types of weapons used, the quality and concentration of any chemical toxin traces, and other facts.

"While Sellstrom may not say who's to blame, there's nothing stopping the secretary-general from interpreting the facts and saying that blame appears to point in a certain direction," the third diplomat said.

The United Nations has repeatedly declined to comment on the expected contents of the report. Syria's UN ambassador, Bashar Jaafari, did not respond immediately to a request for comment.

US threatens military action

Washington says forces loyal to Al-Assad launched the attack on the Al-Ghouta suburbs in Damascus, using the deadly nerve agent sarin — according to US officials — and killing over 1,400 people, many of them children.

Russian President Vladimir Putin, in an opinion piece in The New York Times, confirmed Moscow's view that there is "every reason to believe" the poison gas was used by rebels. That is the Syrian government's position as well.

The attack may have been the worst use of nerve agents since the Iran-Iraq War from 1980 to 1988 when thousands of Kurds were killed by chemical weapons, most notably in the Iraqi town of Halabja.

The United States has threatened to launch military strikes against Syria to deter the government from launching further chemical attacks. But President Obama's administration has said it would allow discussions on a Russian plan to place Syrian chemical weapons under international control to play out before asking the US Congress to vote on authorising the use of force.

Chemical weapons experts say UN investigators should stay out of the blame business.

"They are not going to say: 'This was a rocket used by the Syrian forces,'" said Dieter Rothbacher, a chemical weapons expert who trained members of the UN team while working at the Organisation for the Prohibition of Chemical Weapons (OPCW).

"As an OPCW inspector, you are never supposed to pass judgment. That is up to somebody else. That's up to the guys who take the report and interpret it," said Rothbacher, who helped destroy Saddam Hussein's chemical weapons and co-owns Hotzone Solutions Group, a training and consultancy company.

Western intelligence agencies, including in the United States and Britain, say the evidence already stacks up against Al-Assad, while experts say the rebels do not have the military capabilities to launch a widespread gas attack.

Samples collected by UN inspectors were split, resealed and sent to four partner laboratories, including one in Finland and one in Sweden, UN officials say. The process of analysing them takes weeks, because the biomedical samples, including urine, blood and hair, need time to grow cultures.