US Secretary of State John Kerry has hit out the "brutality" of the Syrian regime of President Bashar al-Assad for its sustained barrel bomb campaign.

In a written statement released on Tuesday, Kerry said use of the bombs was reminding the world of the Assad regime's "true colours."

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"It is the latest barbaric act of a regime that has committed organized, wholesale torture, used chemical weapons, and is starving whole communities by blocking delivery of food to Syrian civilians in urgent need," the statement read.

More than 150 people have been killed in Syria's onetime economic hub and second city over the past four days, in a string of barrel bomb raids and other air strikes, the Syrian Observatory for Human Rights said.

At least eight people, including five children, were the latest victims killed on Tuesday when Syrian army helicopters unleashed a new wave of the bombs which Kerry said were "filled with metal shrapnel and fuel."

The bombs hit a mosque, which the Aleppo media centre said was being used as a school.

Mass exodus

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.

The bombings have caused a mass exodus from rebel-held areas of Aleppo, with many of those seeking refuge finding the way to the Turkish border blocked by fierce infighting between rebel factions.

"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 the war-torn country.

"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.

Aleppo has been divided since the rebels captured large areas of the city in an offensive in the summer of 2012.

Source: Agencies