Dozens of Syrian government troops and Islamic State fighters have been killed in fighting around a government-held air base in eastern Syria in a region that is a stronghold for the jihadists, a monitoring group said on Thursday,

Islamic State used at least two car bombs in its latest assault on the air base near the city of Deir al-Zor, where government troops are holed up, the Syrian Observatory for Human Rights said.

It said at least 18 soldiers and 23 Islamic State fighters had been killed. The base is one of President Bashar al-Assad's last footholds in eastern Syria. There was no mention of the attack on state media.

After more than four years of war, Assad's sway is now mostly confined the cities of western Syria, with the rest held by Islamic State, other insurgent groups, or a Kurdish militia, which controls much of the north.

On Wednesday, Syrian state TV said government troops had quit the Abu al-Duhur air base in the northwesterly Idlib province after a two-year siege by insurgents including the al Qaeda-linked Nusra Front.

Deir al-Zor province borders territories in Iraq that are also controlled by Islamic State, and its oilfields are a major source of revenue for the group.

A U.S.-led coalition has been attacking Islamic State from the air in Deir al-Zor and the neighboring Raqqa province.