Syria's state media has accused opposition fighters of firing a chemical weapon in the north of the country, killing up to 25 people.
The opposition quickly denied the report and claimed regime forces fired the weapon.
Neither of the accusations could immediately be verified but Russia's foreign ministry accused the opposition of being responsible and said that it represented an "extremely dangerous" development.
But White House spokesman Jay Carney said the US had seen no evidence that rebels had used chemicals.
The report by the official SANA news agency marks the first time the government has accused forces seeking to topple President Bashar al Assad of using chemical weapons.
It said "terrorists" fired a rocket "containing chemical materials" into the Khan al Assal area in the northern province of Aleppo on Tuesday. The regime regularly uses the term terrorists to refer to rebels fighting Mr Assad's forces.
It quoted the government's information minister Omran al Zoabi as saying the attack was a "dangerous escalation" in the violent conflict.
He said the firing of the weapon is the "first act" by the opposition interim government announced in Istanbul.
Mr Zoabi added that Turkey and Qatar, which have supported rebels fighting to overthrow President Assad, bore "legal, moral and political responsibility" for the attack, state television reported.
An activist in the area said the opposition had recently seized much of Khan al Assal, including a facility that housed a military academy.
The Aleppo Media Centre, affiliated with the opposition, said there were cases of "suffocation and poison" among civilians in Khan al Assal after a surface-to-surface missile was fired at the area.
It said in a statement the cases were "most likely" caused by regime forces' use of "poisonous gases".
SANA said around 25 people, most of them civilians, were killed and up to 100 more wounded.
An activist in Aleppo province who identified himself as Yassin Abu Raed, not his real name, confirmed the attack and said there were at least 40 cases of suffocation in the area and several deaths.
But he said no details were available as casualties were being taken to a government-controlled area in Aleppo.
President Assad, fighting a two-year uprising against his rule, is widely believed to have a chemical arsenal.
Syrian officials have neither confirmed nor denied having a chemical weapons capability but have said that if it existed it would be used to defend against foreign aggression, not against Syrians.
Western nations have warned Damascus against any use of chemical weapons and have also expressed concern about stockpiles falling into the hands of militant groups.