A Syrian government helicopter has struck a school with a barrel bomb in the northern city of Aleppo, killing at least seven civilians, including children, a monitoring group has said, as other sources reported higher death tolls.

"The primary school students were playing in the courtyard when the barrel bomb hit," Rami Abdel Rahman, who heads the Britain-based Syrian Observatory for Human Rights, said on Sunday.

He said the school is in Saif al-Dawla, a district where control is split between the regime and opposition.

"The children were taking a test there today because they were afraid their own school would be bombarded," Abdel Rahman said.

The centre's director and other civilians are in critical condition after the attack, he added.

The Aleppo Media Centre, an activist group on the ground, said at least 10 people were killed and 20 others injured in the attack.

An AFP correspondent described buildings in ruins and small bulldozers working to clear the rubble.

Classroom walls had completely collapsed, exposing mangled chairs and desks, and men carried whatever books they could save out of the building.

Barrel bombs are highly indiscriminate crude weapons made of old barrels or containers that are packed with explosives and typically dropped from helicopters.

The weapons are often used on rebel-held areas across the country.

Just last month, more than 100 schools in the city had to close temporarily after a regime air attack killed five children and three teachers.

On Saturday, the United Nations issued a fresh call on the Syrian government to stop using explosive weapons in the besieged Palestinian refugee community of Yarmouk and other populated areas.