Syria accused rebels of using chemical weapons in an attack on Tuesday that killed 25 people, but the opposition denied the claim, saying instead regime forces might have used banned weapons.
"Terrorists fired rockets containing chemical materials on Khan al-Assal in Aleppo province," in northern Syria, the official SANA news agency and state television said.
The television report said there were "25 martyrs killed by this toxic product" and that another 100 people suffered injuries, with some in serious condition.
It showed ambulances arriving at a hospital in Aleppo carrying the wounded, with medical officials and residents saying that the attack involved "toxic gas".
One doctor described the use of "toxic chemicals and phosphates causing vomiting and a loss of consciousness".
Moscow said it had information from Damascus that rebels had used chemical weapons, while Washington said there was "no evidence" the insurgents had used the banned arms and warned the regime that using them would be "totally unacceptable".
Britain said it was looking into the reports and that if they were true, it would "revisit" its approach to the two-year conflict.
Syrian Information Minister Omran al-Zohbi called the attack a "dangerous escalation," while state television said 25 people were killed and more than 110 hurt.
"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... (this) crime."
The rebel Free Syrian Army denied its fighters 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 using a long-range missile, and our initial information says it may have contained chemical weapons," FSA spokesman 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."
The Syrian Observatory for Human Rights confirmed a ground-to-ground missile had been fired at an army position in Khan al-Assal, but there was no information on whether it contained chemicals.
It said the attack had killed 16 soldiers and 10 civilians.
In Russia, the foreign ministry said it had information "from Damascus" that rebels had used chemical weapons, but left unclear whether the information came from its own sources or was the same information provided by Syria's regime.
The British Foreign Office said it was looking into the reports.
"The use of chemical weapons would be abhorrent and universally condemned. The UK is clear that use or proliferation of (chemical) weapons would demand a serious response from the international community and force us to revisit our approach so far," said a statement.
In early March, rebel forces took control of Khan al-Assal's police academy in a bloody battle with troops. But regime forces have since taken back much of the town west of Aleppo, Syria's second-largest city.
The international community has expressed repeated concern aver the possibility that Assad's regime would use its chemical weapons against rebels, and there are also fears the stocks could fall into the hands of militants.
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.
Damascus has said it might use its chemical weapons if attacked by outsiders, although not against its own people.