Russia's ambassador to the United Nations says his country has proof Syrian rebels used the nerve gas sarin in March, during a missile attack on a government-controlled suburb of Aleppo.
The rebels have blamed the Syrian army for the attack in Khal al-Assal, which left at least 27 people dead and dozens injured.
The United Nations has not been given free access to the site.
Moscow's permanent representative to the United Nations, Vitaly Churkin, has told reporters the findings were the result of an independent investigation requested by Damascus, the BBC reports.
He says the experts were invited into Khal al-Assal by the Syrian Government and allowed to take their own samples from the missile's point of impact.
Mr Churkin says the samples were analysed at a Russian laboratory that has been certified by the Organisation for the Prohibition of Chemical Weapons.
He says the results show the weapons were not industrially manufactured and were produced in unsophisticated conditions, which gives Russia every reason to believe the nerve gas was created and used by the opposition.
A United Nations spokesperson says the report will be studied.