Islamic State militants launched their biggest attack in months on government-held areas of the Syrian city of Deir al-Zor on Saturday, leaving dozens dead, a war monitor said.

At least six large explosions rocked the city as the militants clashed with government forces, the British-based Syrian Observatory for Human Rights reported.

Syrian government warplanes hit back against Islamic State positions, it added.

Islamic State controls most of Deir al-Zor province, which borders Iraq, including more than half the city, and has besieged the remaining government-held areas of the city for nearly two years.