Syrian rebels and the government have accused each other of firing chemical weapons, reportedly killing at least 25 people in the north of the country.
A Syrian minister said it was a "dangerous escalation" and the "first act" of a new rebel authority.
However, both a chemical weapons monitoring body and the US said there was no evidence they had been used.
Both sides say the attack happened in the Khan al-Assal region north of the second city, Aleppo.
The US says it is looking carefully at the allegations, while Russia has backed the Syrian government's claims.
If confirmed, it would be the first time chemical weapons have been used in the two-year Syrian conflict.
"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".
State TV later said 25 people had died, while the pro-opposition Syrian Observatory for Human Rights put the figure at 26, including 16 soldiers.
Senior rebel and spokesman for the Higher Military Council in Aleppo Qassim Saadeddine said the government had carried out a chemical attack.
"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," he told Reuters news agency.
"Then suddenly we learned that the regime was turning these reports against us. The rebels were not behind this attack."
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.
It said this was "most likely" due to use of "poisonous gases" by government forces.
Ahmet Uzumcu, director general of the Organization for the Prohibition of Chemical Weapons (OPCW), said it was trying "to identify the symptoms which may be detected" in order to make an assessment.
He said he had no more information than what was in the media, Reuters reports.
White House spokesman Jay Carney said the US had "no evidence to substantiate" the allegation that chemical weapons had been used.
He repeated President Barack Obama's warning that any such use would bring "consequences" without being more specific.
Syria's Information Minister Omran al-Zoabi said that Turkey and Qatar, both of which support Syria's uprising, bore "legal, moral and political responsibility" for the attack, state TV reported.
Turkish Prime Minister Recep Tayyip Erdogan strongly denied any links to the alleged use of chemical weapons.
"Turkey has never used chemical weapons and we do not have chemical weapons in our inventory. This is an activity that befits Syria only," the state-funded Anatolia news agency quotes him as saying.
"Syria has attacked and continues attacking its people with chemical weapons," he said.
Russia, a close ally of Syrian President Bashar al-Assad, said it was "very seriously concerned by the fact that weapons of mass destruction are falling into the hands of the rebels".
This "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.
In his first speech after being chosen by the Syrian opposition groups as prime minister of the rebel-held areas, Ghassan Hitto ruled out dialogue with the government.
"We confirm to our people that there is no place for dialogue with the Assad regime," he said in a speech to media and members of the opposition Syrian National Coalition in Istanbul.
Mr Hitto is a Damascus-born IT expert who has lived in the US for many years.
An estimated 70,000 people have been killed and one million have fled Syria since the uprising against President Assad began two years ago.