Under the UN mandate, the inspectors are only authorized to conclude whether chemical weapons have been used in Syria, not assign responsibility for their use, according to Foreign Policy Magazine.

The evidence are based on an examination of spent rocket casings, ammunition, and laboratory tests of soil, blood, and urine samples, on the use of chemical agents.

"I know they have gotten very rich samples -- biomedical and environmental -- and they have interviewed victims, doctors and nurses," said the Western official on Thursday. "It seems they are very happy with the wealth of evidence they got."

French Foreign Minister Laurent Fabius said earlier today that a report by United Nations inspectors will "probably" be published only on Monday. "It will say that there was a chemical massacre," Fabius told French radio.

The inspectors left Syria on August 31 after collecting samples as part of their probe into the attack.

The West and the foreign-backed militants in Syria accused the Syrian government of gassing to death at least 1,400 Syrians on the 21 of August in a Damascus suburb. But the Syrian officials have strongly dismissed the accusation.

The UN Human Rights Chief Navi Pillay has told the 47-nation Human Rights Council that there is little doubt that chemical weapons were used in Syria, but she did not say which of the combatants was suspected of using them.

Pillay opened the council's month-long session telling diplomats that "all the circumstances and responsibilities remain to be clarified." She added that the use of chemical weapons is "one of the gravest crimes that can be committed."

NTJ/NJF