Syrian warplanes have killed more than 330 people in a nine-day bombing campaign, with the US condemning the "indiscriminate" assault and the opposition National Coalition saying it will not attend planned peace talks if it continues.
The vicious campaign has seen aircraft drop barrels of TNT on rebel-held neighbourhoods in Aleppo -- a tactic widely condemned as unlawful -- flooding hospitals with victims, according to activists, medics and others.
The Syrian Observatory for Human Rights, which relies on a network of activists and witnesses on the ground, said Monday that 331 people had been killed since December 15, including 99 children.
It said 30 of the victims died Monday in attacks on the rebel-held Marjeh and Soukkari districts of Aleppo, once Syria's commercial capital.
Activists released footage of what they said was a school targeted in the village of Marea near Aleppo. Children can be seen running from the school and screaming as explosions rumble in the background.
Inside, men pull children from the rubble, their faces caked in dust and blood. It was not possible to verify the footage.
A security source told AFP President Bashar al-Assad's army had adopted the barrel-bombing tactic because of a lack of ground forces, and argued the heavy civilian toll was because the rebels -- branded "terrorists" by the regime -- are based in residential areas.
Aleppo has been split between opposition and government forces since a massive rebel assault in July 2012.
The White House lashed out at the Assad regime for the bombings.
"The United States condemns the ongoing air assault by Syrian government forces on civilians, including the indiscriminate use of SCUD missiles and barrel bombs in and around Aleppo over the last week," said White House spokesman Jay Carney.
Human Rights Watch has accused government forces of using weapons and tactics that fail to distinguish between civilians and combatants, making such strikes "unlawful".
The main opposition National Coalition called on Western states to impose a no-fly zone to halt the attacks, saying they were creating a humanitarian catastrophe and fuelling extremism.
"If the bombing the Assad regime is carrying out and its attempt to annihilate the Syrian people continue, then the coalition will not go to (planned peace talks in) Geneva," said Coalition Secretary General Badr Jamous.
He said Coalition president Ahmad Jarba had been in touch with the British and French foreign ministers to tell them about the attacks.
"If countries cannot put pressure on the regime to stop these operations of destruction... how are they going to pressure the regime at Geneva 2 to obtain a political solution," Jamous said.
The so-called "Geneva 2" talks are aimed at negotiating a political transition to end the war, which has killed an estimated 126,000 people since March 2011 and caused millions to flee their homes.
But the increasingly fractured opposition demands that Assad step down as part of any deal, which the regime rejects.
And several powerful rebel groups have spurned the talks outright, raising concerns that, even if an agreement is reached, it would be unenforceable.
The initiative is aimed at building on the momentum of a deal to eradicate Syria's vast chemical arsenal by mid-2014, which averted punitive US strikes after an August gas attack near Damascus killed hundreds of people.
But Jarba said it is "shameful that... the international community takes measures against chemical arms while... it allows the regime to kill the Syrian people with conventional arms... almost daily".
He called on world powers to take "immediate" decisions to put an end to regime attacks.
Analysts argue that the regime, which has advanced on several fronts recently, has been emboldened by US President Barack Obama's failure to act after Assad allegedly crossed his "red line" against using chemical weapons.
"There are no more red lines, there is a green light," Salman Shaikh, the director of the Brookings Doha Center, told AFP, saying there is an "element of vengeance" in the Aleppo bombings.
"Any credible use of force was taken off the table by Obama and the international community."
Assad said Monday that Syria was being confronted with a major offensive by Islamist extremists.
"The country is facing a takfiri ideology," Assad said, using a term used to describe Sunni Muslim extremists.
"This is terrorism without limits, an international scourge that could strike anywhere and anytime."