Syria's government and rebels have traded accusations of a chemical attack on a northern village near Aleppo.
US officials, however, said on Tuesday there was no evidence of any such attack.
The regime, whose allegation was backed by ally Russia, said 31 people were killed, including 21 civilians and 10 soldiers.
The accusations emerged only a few hours after the opposition to President Bashar al-Assad elected a prime minister to head an interim government that would rule areas seized by rebel forces from the regime.
State-run news agency SANA said more than 100 have been wounded, some of them in critical condition.
SANA published pictures showing casualties, including children, on stretchers in what appears to be a hospital ward.
Syrian Information Minister Omran al-Zoubi called it the "first act" of the newly announced opposition interim government.
Rebels quickly denied the report and accused regime forces of firing the chemical weapon.
Ziad Haddad, a medic in Aleppo, however, told Al Jazeera, the victims seemed to have been exposed to organic pesticides and not chemical weapons, like Sarin and VX nerve agents.
He said several patients arrived in the emergency room earlier on Tuesday morning with cases of suffocation and constricted pupils.
"Several of them died of respiratory inhibition," he said.
"Victims spoke of pungent smell. Chemical weapons are usually odourless."
“Moreover, the number of deaths is small compared to those who would have died had chemical weapons been used."
Haddad said the casualties included Syrian regime soldiers and pro- Assad armed men.
The head of Syria's main opposition group, the Syrian National Coalition, said the group was still investigating the alleged chemical attack near Aleppo.
"Everyone who used it, we are against him, whatever he is," Mouaz al-Khatib told reporters in English in Istanbul.
"We are against killing civilians using chemical weapons, but let us wait some time to have accurate information."
The regime is believed to possess nerve agents as well as mustard gas.
It also possesses Scud missiles capable of delivering them, and some activists said Tuesday's attack was with a Scud missile.
The minister al-Zoubi said the missile containing "poisonous gases" was fired from Nairab district in Aleppo into Khan al-Assal.
The reported attack was in an area just west of the city of Aleppo that had seen fierce fighting for weeks before rebels took over a sprawling government complex there last month.
The facility included several military posts and a police academy that Assad's forces have turned into a military base that regularly fires shells at nearby villages.