(Adds Syrian ambassador, paragraphs 13-15)
By Louis Charbonneau and Anthony Deutsch
UNITED NATIONS/AMSTERDAM, Sept 12 (Reuters) - U.N. chemical weapons investigators will not explicitly blame anyone in their forthcoming report on the Aug. 21 poison gas attack in Syria, but diplomats say their factual reporting alone could suggest which side in the civil war was responsible.
The United States and other Western powers blame forces loyal to Syrian President Bashar al-Assad for the attack. Russian President Vladimir Putin asserts there is "every reason to believe" it was carried out by rebels.
France's U.N. ambassador, Gerard Araud, told reporters Monday is the tentative date for Secretary-General Ban Ki-moon to present chief U.N. investigator Ake Sellstrom's report to the Security Council.
Western diplomats said Ban, who has been highly critical of the Syrian government during the 2-1/2 year civil war, may choose to say whether or not he feels the facts suggest Assad's forces were responsible.
Two Western diplomats said they strongly expect Sellstrom's report will confirm the U.S. view that sarin gas was used in the attack on suburbs of Damascus that killed hundreds of people.
The report could become a bargaining chip in talks between Russia and Western powers on conditions for Syria to give up its chemical weapons and the terms of a U.N. Security Council resolution on the matter.
"We expect it (the report) will have a narrative of evidence," said one U.N. official.
The two Western diplomats said they expected those facts would indirectly point in the direction of the Syrian government. They declined to elaborate.
A third Western diplomat said that while the report will not directly accuse anyone of carrying out the attack due to limitations on Sellstrom's U.N. mandate, it may include facts that suggest blame.
"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 diplomat said.
Foreign Policy's blog "The Cable" cited diplomats on Wednesday voicing similar views - that the facts in Sellstrom's report would suggest the 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.
SYRIA TO ISSUE ITS OWN REPORT
U.N. spokesman Farhan Haq declined to comment on the content of the report, which he said Ban has not yet received. Syria's U.N. Ambassador Bashar Ja'afari told reporters not to speculate about what will be in Sellstrom's report.
"We don't need any anticipation, we don't need 'if,' we need just to judge things as they are," he told reporters after confirming Syria's intention to join the global anti-chemical weapons treaty.
He added in Arabic that the Syrian government would be producing its own report on the Aug. 21 attack, according to an unofficial translation of his remarks.
The United States blames Syrian government forces and says the attack killed more than 1,400 people, many of them children. Syria's government accuses the rebels in the incident.
The attack may have been the worst use of nerve agents since the 1980-1988 Iran-Iraq war when Saddam Hussein's government used chemical weapons against Iranian forces and Kurds, most notably in an attack on the Iraqi town of Halabja that killed thousands of Kurds.
The United States has threatened military strikes against Syria to deter the government from further poison gas attacks. President Barack 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 U.S. Congress to vote on authorizing the use of force.
Chemical weapons experts say U.N. 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 U.N. team while working at the Organization for the Prohibition of Chemical Weapons.
"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'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 Assad, while experts say the rebels do not have the military capabilities to launch a widespread gas attack.
Samples collected by U.N. inspectors were split, resealed and sent to four laboratories, including one in Finland and one in Sweden, U.N. officials say. The process of analyzing them takes weeks because the biomedical samples, including urine, blood and hair, need time to grow cultures. (Additional reporting by Michelle Nichols; Editing by Will Dunham and Mohammad Zargham)