Russia's foreign ministry said on Tuesday it had information that Syrian rebels used chemical weapons that killed 16 people and injured 100 in Aleppo province.
"According to information we are getting from Damascus, early in the morning on March 19, in Aleppo province there was a case of the Syrian opposition using chemical weapons," the foreign ministry said in a statement.
The statement did not make clear whether Russia's information was coming from its own sources or Syrian government officials who have already blamed the rebels for organising their first chemical attack.
The ministry said it was "seriously concerned by the fact that weapons of mass destruction have fallen into the hands of the rebels, which even further complicates the situation in Syria."
The statement once again called on all sides to renounce violence and negotiate to end to the two-year conflict.
Russia is viewed as one of Syria's closest allies and has three times blocked UN sanctions against President Bashar al-Assad's regime.