Russia accused Syrian rebels of using chemical weapons in an attack on Tuesday, calling it an extremely alarming and dangerous development.
"According to information coming from Damascus, a case of the use of chemical weapons by the armed opposition was recorded early in the morning of March 19 in Aleppo province," the Foreign Ministry said in a statement.
It did not specify the exact source of information on the deadly attack, which Syrian President Bashar Assad's government and the rebels blamed on one another. It said the explosion of a piece of ammunition "containing a poisonous substance" killed 16 people and wounded about 100 others.
Syria's information minister said rebels had fired a rocket carrying chemical agents that killed 16 people and wounded 86. State television said later that the death toll had risen to 25.
The pro-opposition Syrian Observatory for Human Rights, which monitors the conflict using a network of contacts in Syria, put the number of dead at 26, including 16 soldiers.
If confirmed, the attack would be the first use of chemical weapons in the two-year-old conflict that has killed more than 70,000 people and set Russia against the West.
Britain's UN envoy said Tuesday that the reports had not yet been "fully verified."
"We have seen those reports; they haven't yet been fully verified," Britain's UN Ambassador, Mark Lyall Grant, told reporters on his way into a UN Security Council meeting on Afghanistan.
"But clearly if chemical weapons were used then that would be abhorrent and it would require a serious response from the international community," he said.
Russia has warned Assad's government not to use chemical weapons and said in December that Damascus had taken steps to ensure that chemical agents were secure by concentrating them at a smaller number of sites.