The mainstream rebel Free Syrian Army accused Moscow of lying on Wednesday, when it said it had evidence that rebel fighters had used sarin nerve gas in an attack.
The FSA "confirms that the Russian report on the revolutionaries' use of sarin gas is false and fabricated", the rebels' political and media coordinator Louay Muqdad told AFP.
Muqdad said the rebels do not have such weaponry and charged that Moscow was "trying to cover for the regime" of President Bashar al-Assad, a longtime ally.
The statement came after Russia's envoy to the United Nations said experts from his country had been to the scene of an attack at Khan al-Assal near the northern city of Aleppo in March and gathered firsthand evidence.
The United States rejected the Russian claims, while France renewed demands that Syria give UN inspectors free access to all sites where chemical arms are alleged to have been used.
"There are more than 10 countries, among them the United States, Britain and France, that have hard, undeniable evidence... that the (Assad) regime used gas and chemical weapons to bomb civilians in Syria," Muqdad said.
"Such bombings, which are prohibited under international law, have targeted at least 15 locations in Syria."
Muqdad said that evidence of the use of chemical weapons had been collected "from the ground, and the bodies of victims, some of which have been transported out of Syria".
Further evidence of the use of sarin by regime forces can be found on "unexploded ordnance launched from warplanes", he said.
Moscow "is trying to cover for the regime, even though it knows that the Free Syrian Army neither has nor wished to have such weapons" he said.
"The use of such weapons would go against our ethics and the goal of our revolution, which is a democratic state that respects human rights."
Russia has rejected claims by the United States and other Western governments that they have intelligence evidence of limited use of chemical weapons by the Damascus regime.
Syria has acknowledged having chemical weapons but insists it would never use them in the civil conflict raging since March 2011.
Sarin, is a deadly nerve gas which was developed by Nazi scientists in 1938 and which was used by now executed Iraqi dictator Saddam Hussein's regime to gas thousands of Kurds in the northern town of Halabja in 1988.