Syrian state media say rebels fighting to topple President Bashar al-Assad have fired a chemical weapon in the north of the country.
"Terrorists launched a missile containing chemical products into the region of Khan al-Assal in the province of Aleppo, killing 15 people, mainly civilians," Sana news agency said.
The government routinely refers to rebels as "terrorists".
Rebels denied the report, accusing the government of using chemical agents.
"We were hearing reports from early this morning about a regime attack on Khan al-Assal, and we believe they fired a Scud with chemical agents," a senior rebel and spokesman for the Higher Military Council in Aleppo, Qassim Saadeddine, told Reuters news agency.
"Then suddenly we learned that the regime was turning these reports against us. The rebels were not behind this attack."'Dangerous escalation'
The Aleppo Media Centre, which is affiliated to the rebels, said there had been cases of "suffocation and poison'' among civilians in Khan al-Assal after a surface-to-surface missile was fired at the area.
But it said this was "most likely" due to use of "poisonous gases" by government forces.
Syria's Information Minister Omran al-Zoabi said that as many as 86 people had been injured in Khan al-Assal, calling the incident a "dangerous escalation" and the "first act" of a newly announced rebel authority.
He also said that Turkey and Qatar, both of which support Syria's uprising, bore "legal, moral and political responsibility" for the attack, state TV reported.
An unnamed Turkish government official denied any links to the reported attack.
"This is a baseless accusation, the Syrian government has accused Turkey in the past as well," the official told Reuters news agency.
Russia, a close ally of Syrian President Bashar al-Assad, condemned the rebels for the alleged use of chemical weapons.
"We are very seriously concerned by the fact that weapons of mass destruction are falling into the hands of the rebels, which further worsens the situation in Syria and elevates the confrontation in the country to a new level," a foreign ministry statement said.
The Syrian government itself has a large stockpile of chemical weapons, and there has been widespread international concern about their security and the possibility that they might be used. More