Syrian army helicopters dropped barrel bombs on Aleppo Tuesday, killing at least eight people as they pressed a bombing campaign launched in mid-December, a monitoring group said.
The ongoing regime campaign against Syria's one-time economic hub drew a harsh comments from US Secretary of State John Kerry, who called it barbaric.
More than 150 people have been killed over the past four days, in a string of barrel bomb raids and other air strikes, the Syrian Observatory for Human Rights said.
The raids have prompted a mass exodus from several rebel-held neighbourhoods in the east of the city.
Five children were among those killed when barrel bombs hit a mosque in the Masakan Hanano district Tuesday, said the Britain-based Observatory, which relies on activists and other witnesses inside the war-torn country.
The Aleppo Media Centre said the mosque was being used as a school.
On Monday, 30 people were killed in barrel bomb attacks and other air strikes, among them 13 children and three women, the Observatory said. Air raids on Saturday killed 85 people.
Civilians trying to flee the bombing have found many escape routes from the city made perilous by fierce fighting between the jihadist Islamic State of Iraq and the Levant (ISIL) and other rebel groups.
Some have made it to the Turkish border. Others have sought refuge in government-held areas of Aleppo, Observatory director Rami Abdel Rahman said.
The Aleppo Media Centre said "people are fleeing in large numbers."
Some neighbourhoods "have become like ghost towns, with shops closed and residents leaving their homes."
The bombing campaign has accompanied ground assaults by government forces that have seen them recapture some territory south and east of Aleppo, including the international airport, in recent months.
Kerry said in a statement that "each and every day that the barrel-bombing of Aleppo continues, the Assad regime reminds the world of its true colours.
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"It is the latest barbaric act of a regime that has committed organised, wholesale torture, used chemical weapons, and is starving whole communities by blocking delivery of food to Syrian civilians in urgent need."
Aleppo has been divided since the rebels captured large swathes of it in the summer of 2012, and much of its historic Old City has been levelled in the fighting.
"Given this horrific legacy, the Syrian people would never accept as legitimate a government including Assad," Kerry said, referring to peace talks due to resume in Geneva next week aimed at installing a transitional government in Syria.
"While the opposition and the international community are focused on ending the war... the regime is single-mindedly focused on inflicting further destruction to strengthen its hand on the battlefield and undermining hopes for the success of the Geneva II process," Kerry said.
Suicide bomber strikes Islamist HQ
Elsewhere in the country, the Observatory said an ISIL fighter blew himself up at the headquarters of an Islamist rebel brigade in the eastern province of Deir Ezzor.
There were no immediate details of casualties in the incident, in the town of Mayadeen.
The group has used the tactic on multiple occasions in its conflict with Islamist and moderate rebels, who began fighting it in January.
Some in Syria's opposition welcomed jihadist fighters early in the conflict, valuing their superior weapons and battlefield experience.
But multiplying allegations that the group was kidnapping and killing civilians and rival rebels prompted a backlash that has seen ISIL lose ground in Aleppo and Idlib province.
However, ISIL has cemented its control over Raqa, the only provincial capital to fall from regime hands.
The Observatory said Tuesday the group distributed fliers there admitting "unacceptable" behaviour by some of its members.
"We apologise to each and every person subjected to any harassment, and we ask God to forgive us," the flyer reads, cautioning members of the group to promulgate religion with "kindness" rather than "harshly."
And it urges citizens in Raqa who feel they have been mistreated by ISIL fighters to refer them to Islamic courts there.