Syrian regime air strikes killed at least 52 civilians, including seven children, in strongholds of the Islamic State jihadist group, a monitoring group said Friday in a new toll.

The raids struck Al-Bab and Qbasin Thursday in the northern province of Aleppo, according to the Syrian Observatory for Human Rights, which relies on a network of activists on the ground.

Previously the toll had stood at 37 dead.

"At least 52 civilians, including seven children, three teenagers and two women, were killed in the Syrian army air raids," said the Observatory, adding that dozens were seriously wounded.

The regime air force has killed thousands of people since it was first deployed in the war in July 2012.

Activists accuse the government of killing more civilians than jihadists in the raids.

The UN and international rights groups have repeatedly called on the government to refrain from using its air force against inhabited areas.

The country's multi-sided civil war has killed an estimated 200,000 people and displaced half of its population.

A US-led military coalition is carrying out regular air strikes against the Islamic State group, which has seized large areas in Syria and neighbouring Iraq.

On Thursday, about 60 jihadists were killed in fighting with Kurdish forces for control of territory in northern Syria, according to the Observatory.