BEIRUT (Reuters) - 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 of dead, a war monitor said.

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

Syrian government warplanes hit back against Islamic State positions, the Observatory and state media said.

Syrian state news agency SANA said the army was repelling attacks by the ultra-hardline jihadist group along several frontlines, including around the military airport controlled by the government.

Islamic State shelling killed three people and wounded nine in residential government-held districts, SANA reported.

The fighting and bombardment killed at least 12 government forces and 20 Islamic State fighters, the Observatory said.

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.

The Syrian government and its Russian ally have made regular aid drops into the encircled zone, where about 200,000 people live, lacking food and medicine.

Deir al-Zor province links Islamic State's de facto capital in the Syrian city of Raqqa with territory controlled by the militant group in Iraq.

(Reporting by Ellen Francis, Editing by Angus MacSwan)