Moscow's UN ambassador, Vitaly Churkin, said Russian experts visited the location where the projectile struck and took their own samples of material from the site.
Those samples, he said, were then analyzed at a Russian laboratory certified by the Organization for the Prohibition of Chemical Weapons.
"It was established that on March 19 the rebels launched an unguided Basha'ir-3 projectile towards Khan al-Assal controlled by the government forces," he said.
"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," Churkin said.
"Hexogen, utilized as an opening charge, is not utilized 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."
Churkin said he had informed UN Secretary-General Ban Ki-moon of the Russian findings.
The incident at Khan al-Assal in the northern province of Aleppo killed more than two dozen people.
Ban is scheduled to meet Ake Sellstrom, the Swedish scientist heading a UN team established to investigate allegations of chemical weapon use in Syria, in New York this week.
Syrian UN Ambassador Bashar Ja'afari on Monday said his government has invited Sellstrom and UN disarmament chief Angela Kane to Damascus to discuss allegations of banned arms use in Syria's conflict.
The conflict in Syria started in March 2011, when pro-reform protests turned into a massive insurgency following the intervention of Western and regional states.
The unrest, which took in terrorist groups from across Europe, the Middle East and North Africa, has transpired as one of the bloodiest conflicts in recent history.
As the foreign-backed insurgency in Syria continues without an end in sight, the US government has boosted its political and military support to Takfiri extremists.
Washington has remained indifferent about warnings by Russia and other world powers about the consequences of arming militant groups.