More than 80 people are reported to have been killed in a fresh aerial bombardment of the northwestern Syrian city of Aleppo.

Monday's attacks followed on from Sunday's air raids, which targeted Aleppo's Masaken Hanano neighbourhood, hitting a second-hand market, a two-story building and a main road, activists said, in the eighth day of government airstrikes on the city.

"The medics say they are removing people in parts; they aren't sure how many there are," Hassoun Abu Faisal, an activist with the group the Aleppo Media Centre told Associated Press news agency.

A school in Marea on the outskirts of Aleppo has been one of the targets in the latest attacks [AFP]

He said the bombs destroyed vehicles lining a main road and a two-story building, leaving a crater where part of the market was.

Activists said the airstrikes were carried out by government helicopters that dropped barrel bombs, crude devices filled with explosives and fuel that are wildly inaccurate but cause massive damage on impact.

The bombs are explosive-filled cylinders or oil drums that are often rolled out of the back of helicopters with little attempt at striking a particular target. They are capable of causing widespread casualties and significant damage.

Human rights groups have said that even if Syrian forces are targeting rebels with the bombs, they often explode in residential areas, killing civilians.

In the town of Marea, north of Aleppo, a barrel bomb that exploded near a school used by Syrians fleeing fighting in other areas killed three members of the same family, according to Abu Faisal and the Observatory.

Attacks near Homs school

Schools decided to close Monday in opposition-held parts of Aleppo to avoid more casualties, citing Sunday's attacks in Marea, the Aleppo Media Centre reported.

Syrian officials have not commented on the air raids in Aleppo, the country's largest city, and a major front in the war since the rebels launched an offensive there in mid-2012.

The city has been carved into opposition- and government-held areas.

Meanwhile, at least 20 people were killed after a suicide bomber exploded a device near a primary school in the predominantly Shia town of Umm al-Amed in Homs province.

Seven of the dead were children, according to the Observatory.

Syria's state news agency also put the death toll at 20 but said six children had died.

An estimated 6,627 children have been killed within the past three years, a UN monitoring group believes.

More than 100,000 people have been killed in the conflict, in a war where no one seems to have the upper hand.