Syrian government forces have sent reinforcements to an air base being attacked by Islamic State militants in northeast Syria where no fewer than 30 of the radical group's fighters were killed on Thursday, a group monitoring the violence said, according to Reuters.

The airbase at Tabqa, some 40 km (25 miles) east of the city of Raqqa in northeast Syria, represents the government's last foothold in an area otherwise controlled by the Islamic State group that has seized swathes of territory in Syria and Iraq.

The Syrian Observatory for Human Rights, which says it gathers material from all sides in the Syrian conflict, said the reinforcements had been flown in overnight into Friday.

It said no fewer than 30 Islamic State fighters had been killed and dozens more wounded on Thursday, August 21 by heavy bombardment and landmines in areas surrounding the base.

The city of Raqqa on the Euphrates river represents the Islamic State's stronghold in Syria. Boosted by weaponry seized in Iraq, the group has taken three Syrian military bases in the area in recent weeks.

The Syrian state news agency SANA on Thursday denied reports that Islamic State fighters had entered the air base. It also said government forces had taken a nearby village.

Islamic State, which has been hit by U.S. air strikes in Iraq, controls roughly a third of northern and eastern Syria.