Russia on Tuesday handed over evidence to the United Nations indicating Syrian rebels used sarin gas in an attack in March, its UN envoy said.
Ambassador Vitaly Churkin said Russian experts had been to the scene of the attack at Khan al-Assal near Aleppo and gathered firsthand evidence.
The Russian action risks reigniting an international dispute over the use of chemical weapons in the 26-month-old war however.
The United States rejected the Russian claims. France renewed demands that Syria give UN inspectors free access to all sites where chemical arms are alleged to have been used.
The Syrian government has refused to let a UN inspection team into the country, but this week invited UN officials for talks on the investigation.
Churkin told reporters a Russian inquiry had "established" that rebel forces fired a Bashair 3 missile at Khan al-Assal on March 19, killing 26 people, including 16 troops.
"The results of the analysis clearly indicate that the ordinance used in Khan al-Assal was not industrially manufactured and was filled with sarin," added Churkin, who said he handed over an 80-page report to UN leader Ban Ki-moon.
"There is every reason to believe that it was the armed opposition fighters who used chemical weapons in Khan al-Assal," the Russian ambassador said.
Churkin added that Russia has information that the Bashair 3 missiles were produced by the Bashair al-Nasr group which he said was affiliated to the Syrian Free Army.
UN spokesman Martin Nesirky said the Russian report is being "studied".
Britain, France and the United States say they have handed over evidence to UN experts that President Bashar al-Assad's forces have used chemical weapons in the conflict.
Britain and the United States have between them handed over information on 10 alleged chemical arms attacks by government forces, according to a UN diplomat.
Following Churkin's statement, the United States reaffirmed the insistence by western nations that they have no evidence linking the Syrian opposition with chemical weapons.
"We have yet to see any evidence that backs up the assertion that anybody besides the Syrian government has the ability to use chemical weapons, (or) has used chemical weapons," White House spokesman Jay Carney said.
Carney said the Syrian government had to allow access to UN inspectors. The same message was also given by the French ambassador to the United Nations, Gerard Araud, when asked about the Russian report.
Syria has said it will let UN experts investigate only the Khan al-Assal attack, but Ban has insisted on "unfettered" access to all sites where there have been allegations of chemical attacks.
The government this week invited Ake Sellstrom, a Swedish expert who heads the UN investigation panel, and Angela Kane, UN High Representative for Disarmament, to Damascus for talks.
Syria's UN ambassador Bashar Jaafari said Monday the aim was to "discuss further the mechanism and terms of reference" of the UN inquiry set up by Ban in March.
Churkin said the Russian report would be sent to the Western powers. He reaffirmed that Russian experts were "not impressed" by the evidence provided by Britain, France and the United States.
The United Nations has said the material handed over by the Western countries cannot be considered definitive proof because the origin of the evidence has to be verified by UN experts.
"Unlike other reports provided to the secretary general, our samples were taken at the projectile impact point by Russian experts in person. They were not transmitted to us through third points," Churkin said.