Russia says it has evidence showing a projectile that hit a northern Syrian village contained the nerve agent sarin and was most likely fired by rebels.
Moscow's permanent representative to the UN told reporters that the findings were the result of an independent investigation requested by Damascus.
Both sides have accused each other of attacks involving chemical weapons.
The incident in Khan al-Assal, outside Aleppo, on 19 March left at least 27 people dead and dozens injured.
Last month, the UN Human Rights Council's Independent International Commission of Inquiry said there were reasonable grounds to believe that "limited quantities of toxic chemicals" had been used at Khan al-Assal, as well as in three other attacks.
But it had "not been possible... to determine the precise chemical agents used, their delivery systems or the perpetrator", it added.
The inquiry also called on the Syrian authorities to allow a team of UN chemical weapons experts into the country - a request Damascus has so far refused because of disputes over access to areas that the UK and French governments also want investigated.
'No credible reporting'
On Monday, Russia's ambassador to the UN, Vitaly Churkin, revealed that its Syrian ally had asked Russian experts to look into the Khan al-Assal attack.
They had visited the location where the projectile landed and taken their own samples, which were then analysed at a Russian laboratory certified by the Organisation for the Prohibition of Chemical Weapons (OPCW), Mr Churkin said.
The chemical agent was carried by a "Bashair-3 unguided projectile", which was produced by the Bashair al-Nasr Brigade, a rebel group affiliated with the Free Syrian Army, he alleged.
"The results of the analysis clearly indicate that the ordnance used in Khan al-Assal was not industrially manufactured and was filled with sarin."
"The projectile involved is not a standard one for chemical use," Mr Churkin added. "Hexogen, utilised as an opening charge, is not utilised in standard ammunitions. Therefore, there is every reason to believe that it was armed opposition fighters who used the chemical weapons in Khan al-Assal."
Mr Churkin said the report had been submitted to UN Secretary General Ban Ki-moon, whose spokesman said he took "seriously all credible allegations".
The UK and France sent letters to the secretary general in late March which reportedly detailed evidence based on witness interviews and soil samples that chemical weapons had been used on multiple occasions, including at Khan al-Assal.
In mid-June, the US said its intelligence agencies believed government forces had used chemical weapons, including sarin, "on a small scale against the opposition multiple times in the last year", resulting in an estimated 100-150 deaths.
Following Mr Churkin's announcement, a UK government spokesman told the BBC: "We will examine whatever is presented to us, but to date we have seen no credible reporting of chemical weapons use by the Syrian opposition, or that the opposition have obtained chemical weapons."
White House spokesman Jay Carney said the US had also "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".
"Our ability as an international community to investigate the use of chemical weapons in Syria is hampered by Assad's refusal to allow a United Nations investigation."
Sarin is considered 20 times as deadly as cyanide and is impossible to detect due to its odourless, tasteless and colourless properties. The agent attacks the nervous system, often causing respiratory failure and can cause death within minutes of exposure.