At least 11 people, including two children, were killed Tuesday in Syrian government air strikes as they fled a northern town held by the Islamic State group, a monitor said.

The Britain-based Syrian Observatory for Human Rights also reported 12 civilians and 15 IS fighters were killed in air strikes and shelling in the eastern city of Deir Ezzor.

The monitor said the group of 11 were fleeing the embattled IS-held town of Al-Bab, near the northern border with Turkey, when they were hit in a government air strike.

The dead included at least 10 civilians, among them two children, but the identity of the 11th person killed was unclear.

The strike hit the group as they reached the nearby village of Qasr al-Bureij, also under IS control, the Observatory said.

Al-Bab in the northern province of Aleppo has come under heavy assault in recent weeks, with Turkish, Russian and Syrian warplanes carrying out strikes in or around the town.

IS is also fighting fierce battles in Deir Ezzor city, which the jihadist group has besieged since early 2015.

It already controlled half the city, but has made further advances in recent days, prompting fierce fighting and heavy air strikes by both Syria and its Russian ally.

The Observatory said air strikes by Syrian and Russian warplanes killed 12 civilians and 15 IS fighters, but the toll could rise further.

Deir Ezzor is the capital of the oil-rich province of the same name which borders Iraq.

The fighting has forced the World Food Programme to suspend air drops of aid to besieged civilians in the city, and the UN has warned food supplies could run out within weeks.

More than 310,00 people have been killed in Syria since the conflict began with anti-government protests that were met with a regime crackdown.

A truce brokered by Russia and rebel backer Turkey has been in place since December 30, but it excludes IS.