The ministry said the attack represented an "extremely dangerous" development in the two-year Syrian crisis. It said the rebels detonated a munition containing an unidentified chemical agent early Tuesday in the province of Aleppo. It didn't give further details.
Syria's state-run news agency has also accused the rebels of the attack, saying 25 people died, but the rebels say government forces fired the weapon.
Russia said that the evidence of rebels getting hold of weapons of mass destruction had taken the confrontation to a new stage.
Russia has been the main ally of President Bashar Assad's regime since the start of the uprising, in which more than 70,000 have died.