The US and UN are attempting to verify claims and counterclaims by Syrian troops and insurgents that chemical weapons were used for the first time in the two-year conflict.
NATO supreme commander Admiral James Stavridis, decrying the deteriorating situation, said in Washington on Tuesday that NATO members are mulling plans for possible military action in Syria.
However he stressed that any intervention would only occur with a UN Security Council resolution and agreement from the alliance's 28 members.
His comments came after Syrian state media reported that "terrorists fired rockets containing chemical materials on Khan al-Assal in Aleppo province," with Information Minister Omran al-Zohbi calling 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."
In Washington, White House spokesman Jay Carney said the US 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 be seeking clarification from the Russian authorities.
United Nations Secretary General Ban Ki-moon "remains convinced that the use of chemical weapons by any party (in Syria) under any circumstances would constitute an outrageous crime," the UN said.
Ban and Ahmet Uzumcu, Director General of the Organization for the Prohibition of Chemical Weapons, "shared deep concern about the alleged use of chemical weapons in Syria," the UN said in a statement.
UN spokesman Martin Nesirky said that the UN was "aware of the report" that chemical weapons had been used in Syria, but said "we are not in a position to confirm it."
Britain said that if the reports of chemical weapons usage were true it would "revisit" its approach to the two-year conflict.
Syrian 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.
NATO chief Stavridis told the US Senate Armed Services Committee that some NATO members had mooted the option of targeting Syria's air defense network, a strategy successfully used in Libya in 2011.
"We are looking at a wide range of operations, and we are prepared, if called upon, to be engaged as we were in Libya," Stavridis said, adding that the Syrian conflict was deteriorating.
NATO's sole involvement in Syria's brutal civil war to date has been to position Patriot missile batteries along the Turkish border in order to prevent any air or missile launches from the Syrian side.
'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 amid allegations that the Muslim Brotherhood had backed his candidacy.
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.
US lawmakers heard Tuesday that women and children are suffering the worst horrors in Syria, with many subjected to rape and sexual violence in a war that has traumatized a generation of young people.
"It is hard to truly capture the horror of going to the camps and being shown pictures by a woman of her dead children on her cell phone in this very tragic twist on technology," assistant administrator at the US Agency for International Development Nancy Lindborg told the lawmakers.