Iran on Wednesday accused "armed opposition groups" in Syria of having used chemical weapons on the city of Aleppo, warning against a recurrence of such attacks, the official IRNA news agency reported.
"The Islamic republic of Iran strongly condemns the inhumane act by armed opposition groups in using chemical weapons in the city of Aleppo," foreign ministry spokesman Ramin Mehmanparast said.
"Responsibility if there is a recurrence of such crimes will also fall on countries supporting" the rebels, he added.
Syrian officials and media said 31 people had been killed in the attack, and that about 100 more were injured.
The ministry also summoned the Swiss charge d'affaires and told him "the American government has a responsibility to stop Syrian rebels from acquiring and using such weapons," IRNA said, quoting ministry official Mohsen Naziri.
Switzerland represents US interests in Iran, which severed diplomatic relations with Washington in 1979.
US President Barack Obama, meanwhile, told a news conference in Israel that he was highly doubtful about claims the rebels had used chemical weapons, and warned the Syrian regime would be held accountable for any use of such weapons.
The use of chemical agents against the Syrian people would be a "grave and tragic mistake" and a "game-changer," Obama said.
"The Assad regime must understand they will be held accountable," he said.
The US envoy to Damascus, Robert Ford, told lawmakers at a House of Representative hearing there was no proof of a chemical weapons attack in Aleppo province.
"So far we have no evidence to substantiate the report that chemical weapons were used yesterday," Ford said on Wednesday.
Syria and the rebels on Tuesday accused each other of using chemical weapons for the first time since an uprising against the regime of President Bashar al-Assad began in March 2011.
Iran staunchly supports Assad, and regards many Syrian opposition groups as "terrorists" being backed by Western and Arab countries.
Russia, another Assad ally, said on Tuesday it had "information" from Damascus that rebels had used chemical weapons, but on Wednesday Moscow backtracked and said the information still had to be checked.
UN Secretary General Ban Ki-moon on Tuesday warned that the use of chemical weapons by any party in Syria would "constitute an outrageous crime."