Troops and insurgents Tuesday accused each other of using chemical weapons for the first time in Syria's two-year conflict, as the newly elected rebel premier ruled out dialogue with the regime.
State media reported that "terrorists fired rockets containing chemical materials on Khan al-Assal in Aleppo province," and Information Minister Omran al-Zohbi called the attack a "dangerous escalation."
Deputy Foreign Minister Faisal Muqdad said 31 people had been killed, and state media added that around 100 more were injured.
The insurgents denied the charges and accused regime forces of a deadly long-range missile attack that caused "breathing problems".
The Russian foreign ministry said it had "information" from Damascus that insurgents used chemical weapons, and expressed concern such weapons falling into the hands of rebels "complicates further the situation in Syria."
Responding to claims by the Syrian government and its ally Russia, White House spokesman Jay Carney said Washington has "no evidence to substantiate the charge that the opposition has used chemical weapons."
State Department spokeswoman Victoria Nuland said US ambassador to Moscow, Michael McFaul, would also be seeking clarification from Russian authorities.
Britain, meanwhile, said if reports of chemical weapons usage 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 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," 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."
The Syrian Observatory for Human Rights said a ground-to-ground missile had been fired at an army position in Khan al-Assal, killing 16 soldiers and 10 civilians.
The watchdog was unable to say if the missile contained chemical materials.
The international community has expressed repeated concern that Assad's regime might use chemical weapons against the rebels, and also that they could fall into the hands of militants.
Some 70,000 people have been killed in over two years of fighting in Syria, with millions displaced by the fighting. The Observatory said at least 112 people were killed on Tuesday alone in violence throughout the country.
-- 'No dialogue' with Assad --
Meanwhile, Ghassan Hitto, elected the first rebel premier early Tuesday in Istanbul, pledged 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.
He was chosen by a majority of the main opposition Syrian National Coalition, after hours of consultations and amid allegations that the Muslim Brotherhood had backed his candidacy.
Coalition chief Ahmad Moaz al-Khatib had proposed talks with regime officials, with conditions, including the release of some "160,000 detainees."
The United States welcomed Hitto's election but Russia's foreign ministry greeted the news with "deep regret," warning it would "only deepen the state of internal instability in Syria."
Hitto said he would focus on regime change, but also seek to bring security and basic services to large swathes of rebel-held territory in northern Syria which are mired in poverty and insecurity.
"The main priority we have before us is to make use of all tools at our disposal to bring down the Assad regime," he said, adding that he will work with rebels to ensure "security and the rule of law."
His interim government would "fight crime" and "limit the proliferation of weapons" in rebel-held regions, he said.
Hitto also promised to coordinate with international humanitarian agencies to bring in much-needed aid, and "run border controls" that have fallen into rebel hands.
A former IT executive who has lived in the United States for many years, Hitto said he form a government of technocrats.
Free Syrian Army chief of staff Selim Idriss has said the rebels will work under the umbrella of the provisional government.