A report by the world's chemical weapons watchdog that the banned nerve agent sarin was used in an attack in northern Syria in April is based on "doubtful evidence", Russian news agencies quoted Russia's Foreign Ministry as saying on Friday.
The report, seen by Reuters on Thursday, was circulated to members of the Organisation for the Prohibition of Chemical Weapons (OPCW) in The Hague, but was not made public.
The attack on April 4, when dozens of people were killed in the town of Khan Sheikhoun in northern Idlib province, was the most deadly in Syria's civil war in more than three years. It prompted a U.S. missile strike against a Syrian air base which Washington said was used to launch the strike.
"Unfortunately, after a first reading of this document we are forced to note that its conclusions are based on extremely doubtful evidence," TASS news agency quoted Russia's Foreign Ministry as saying.
"The contents of the report compiled by a special commission of the OPCW, are largely biased, which makes us think that the activities of this structure serve a political order," TASS quoted the ministry as saying.
Russia and its allies in the administration of Syrian President Bashar al-Assad deny that his forces deployed chemical weapons in Khan Sheikhoun.
Moscow has said the attack was carried out by Assad's opponents, who, Russian officials alleged, made it look as though it was the work of government forces.