Syria's government and rebels have accused each other of launching a deadly chemical attack near the northern city of Aleppo.
There has been no immediate independent confirmation of the claims.
Syria's state-run SANA news agency said 25 people were killed and dozens were wounded in the alleged rebel attack on March 19.
Information Minister Omran al-Zoabi said rebels fired "a rocket containing poison gases" at the town of Khan Al-Assal, near Aleppo, from a rebel-held district of the city.
Syrian state television broadcast footage of what it said were casualties of the attack arriving at a hospital.
A spokesman for the rebel High Military Council in Aleppo, Qassim Saadeddine, denied this. He accused government forces of firing "a Scud [missile] with chemical agents."
The head of the opposition Syrian National Coalition, Moaz Al-Khatib, said the opposition is looking into the reports but that it opposes any use of chemical weapons, by either side.
Several international organizations, including the Organization for the Prohibition of Chemical Weapons, the World Health Organization, and the International Committee of the Red Cross said they had no independent information on the alleged chemical attack.
In Washington, White House spokesman Jay Carney said the United States has no evidence to back the Syrian government's claim that rebels used chemical weapons.
Carney said it is a serious concern for the United States that the Assad regime could use such weapons.
Russia, meanwhile, appeared to back the Syrian government's claim.
The Russian Foreign Ministry said "information coming from Damascus" on the use of chemical weapons by rebels was "extremely dangerous."
The allegations came hours after the opposition Syrian National Coalition elected a prime minister of an interim government to be based in rebel areas in northern Syria.
In a speech following his election in Istanbul on March 19, Ghassan Hitto ruled out any dialogue with the Syrian government.
"You people who astonished the world with your courage and determination will be saved from slavery, from an oppressive system that the modern world has never seen before," he said. "It killed the people and destroyed the country."
Analysts say it is unclear whether rebels fighting Assad's forces in Syria will accept the interim government's authority.
The United Nations says more than 70,000 people have been killed in the 2-year-old conflict in Syria. About 1 million have been forced to flee the country.
With reporting by Reuters, AP, and the BBC