Syria's government and rebel forces accused each other of using chemical weapons for the first time on Tuesday, as a newly elected rebel prime minister ruled out dialogue with President Bashar al-Assad's regime.
"Terrorists fired rockets containing chemical materials on Khan al-Assal in Aleppo province," the state news agency SANA and Syrian state television reported.
Syria's Information Minister Omran al-Zohbi called the attack a "dangerous escalation," while state television said 25 people were killed and around 100 injured in the incident.
A spokesman for the Syrian rebels denied its fighters had used banned weapons, instead blaming Assad's regime for a deadly long-range missile attack that caused "breathing problems".
Key Assad ally Moscow said it had "information" from Damascus that rebels had used chemical weapons, while Washington said there was "no evidence" the insurgents had staged their first chemical attack and warned it would be "totally unacceptable" for the regime to use such arms.
Britain said it was looking into the reports and that if they were true, it would "revisit" its approach to the two-year conflict.
State television showed ambulances arriving at a hospital in Aleppo carrying the wounded, with medical officials and residents saying that the attack involved "toxic gas".
"We have neither long-range missiles nor chemical weapons. And if we did, we wouldn't use them against a rebel target," mainstream rebel Free Syrian Army spokesman Louay Muqdad said.
"We understand the army targeted Khan al-Assal using a long-range missile, and our initial information says it may have contained chemical weapons," he said.
"There are many casualties and many injured have breathing problems," Muqdad said in Istanbul, where Syria's opposition has gathered to pick a rebel prime minister.
The Syrian Observatory for Human Rights, a Britain-based watchdog, confirmed that 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 chemical material.
It said the attack had killed 16 soldiers and 10 civilians.
The international community has expressed repeated concern over the possibility that Assad's regime would use its chemical weapons against the insurgents, and there are also fears the stocks could fall into the hands of militants if the regime loses control over them.
The Syrian conflict, now entering its third year, has killed some 70,000 people and forced millions to flee from their homes, according to the United Nations.
Meanwhile, the rebels' new premier, Ghassan Hitto, used his inaugural speech in Istanbul to reiterate there would be no dialogue with Assad's regime.
"We confirm to the great Syrian people that there will be no dialogue with the Assad regime," said Hitto, who is now tasked with setting up an interim government to administer rebel-held areas in the strife-torn country.
Hitto was chosen early Tuesday by a majority of the main opposition National Coalition members, after hours of consultations.
But the vote was not without controversy, as several coalition members accused the powerful Muslim Brotherhood of backing his candidacy.
The election came some two months after coalition chief Ahmad Moaz al-Khatib proposed talks with regime officials with conditions, including that some "160,000 detainees" be released.
"The regime ended that proposal (for talks), not the opposition," coalition chief Khatib said.
In a speech laying out the new government's priorities, Hitto called the regime "a gang" that "destroyed the country".
"The main priority we have before us is to make use of all tools at our disposal to bring down the Assad regime," the 50-year-old said, while pledging to offer "all possible assistance" to residents living in areas free from army control.
The opposition intends to help administer daily life in large swathes of rebel territory mired in poverty and insecurity.
Hitto said his interim government, which opponents believe should be formed within a month, would "collaborate with the Free Syrian Army" to ensure "security and the rule of law" for civilians.
He said the interim government would "fight crime" and "limit the proliferation of weapons" in areas from which the army has withdrawn, but which are plagued with insecurity, kidnappings and theft.
Syria's first rebel premier also said the new government would coordinate with international humanitarian agencies to bring in much-needed aid, and "run border controls" that have fallen into rebel hands.
Ministers in the government will be chosen based on their technical and professional capabilities rather than political interests, added Hitto, a former IT executive who spent years living in the United States.
Free Syrian Army chief of staff Selim Idriss has already said that the insurgents would work under the umbrella of the provisional government.
Supporters have praised Hitto's knack for building diplomatic ties that have been key to securing financial support for Syrians displaced by the conflict.