Syria's government on Tuesday accused rebel forces of using chemical weapons for the first time, but the opposition denied the claim, saying instead that government forces might have used banned weapons.
"Terrorists fired rockets containing chemical materials on Khan al-Assal in Aleppo province," the state news agency SANA and Syrian state television said.
Syria's Information Minister Omran al-Zohbi called the attack a "dangerous escalation," saying that 16 people were killed and 86 injured in the incident.
"The international community and the states that arm, finance and shelter the terrorists should (take note) of the crime committed today in which terrorists used a weapon that is prohibited by international law," he said.
He said countries that backed the rebels, including Turkey and Qatar "bear the legal, moral and human responsibility for the crime that left 16 dead and 86 injured, both civilians and soldiers," Zohbi added.
A spokesman for the Free Syrian Army in Istanbul denied rebel forces had used chemical weapons, blaming President Bashar al-Assad's regime for a deadly rocket attack that caused "breathing problems".
"We understand the army targeted Khan al-Assal (in Aleppo province) using a long-range missile, and our initial information says it may have contained chemical weapons," Louay Muqdad told AFP.
"There are many casualties and many injured have breathing problems," he said in Istanbul, where Syria's opposition has gathered to pick a rebel prime minister.
"We have neither long-range missiles nor chemical weapons. And if we did, we wouldn't use them against a rebel target," said Muqdad.
The Syrian Observatory for Human Rights, a Britain-based watchdog, confirmed that a ground-to-ground missile had been fired at a Syrian army position in Khan al-Assal, but there was no information on whether it contained chemical material.
The group said the incident had killed 16 soldiers and 10 civilians.
"I'm not able to confirm if the missile contained chemical materials or not," the group's director Rami Abdel Rahman said.
In early March, nearly 200 regime forces and rebels were killed in fighting over control of a police academy in Khan al-Assal, which is west of Aleppo.
Rebel forces seized the academy, but in the past week regime forces retook much of the town in a counter-offensive, according to the Observatory.
The international community has expressed repeated concern aver the possibility that Assad's regime would use its chemical weapons against rebel forces, and there are also fears the stocks could fall into the hands of militants if the regime loses control over them.
Syria's chemical weapons stockpile, which dates back to the 1970s, is the biggest in the Middle East but its precise scope remains unclear according to analysts, and the regime has not acknowledged having the arms.
The country has hundreds of tons of various chemical agents, including sarin and VX nerve agents, as well as older blistering agents such as mustard gas, dispersed in dozens of manufacturing and storage sites, experts say.
But it remains unclear if the chemical weapons are mounted and ready to be launched on Scud missiles, if the chemical agents are maintained effectively, and whether the regime is able to replenish its chemical stocks.
Damascus has said it might use its chemical weapons if attacked by outsiders, although not against its own people.
Please follow Military & Defense on Twitter and Facebook.