Beirut: Islamic State of Iraq and the Levant (Isil) militants stormed an air base in northeast Syria on Sunday, capturing parts of it from government forces after days of fighting over the strategic location, a monitoring group said.

The air base at Tabqa, some 40km east of the city of Raqqa, is the Syrian army’s last foothold in an area otherwise controlled by the Isil group that has seized large areas of Syria and Iraq in recent months.

There were fierce clashes between militants and government forces within the walls of the air base according to the Syrian Observatory for Human Rights, which tracks the violence in Syria using sources on the ground. Earlier on Sunday the Syrian air force had bombed areas around the base.

At least 100 militants have been killed since Tuesday when the group first attacked the base, and more than 300 have been injured, the Observatory said, adding that at least 25 Syrian army soldiers had also died.

Syrian media did not carry any immediate reports on the latest assault. On Saturday Syrian state television and the national news agency showed measures used to protect the base and said Isil had suffered heavy losses in its attacks.

It showed footage of bodies it said were militants.

The Syrian army sent reinforcements to the base overnight on Friday to fight Isil, which controls roughly a third of northern and eastern Syria.

The city of Raqqa on the Euphrates river is Isil’s stronghold in Syria. The group, a radical offshoot of Al Qaida, has taken three Syrian military bases in the area in recent weeks, boosted by arms seized in Iraq.

To the west, the group withdrew from areas it controlled outside the Syrian city of Homs on Sunday and retreated east after coming under attack from rival Islamist fighters, the Observatory said.

Militants from the group withdrew from a headquarters north of Homs on the orders of their leader, Abu Bakr Al Baghdadi, the Observatory said, citing sources in areas north of Homs.

They said Isil gave up the territory to Al Nusra Front, Al Qaida’s official wing in Syria.

As well as Al Nusra Front, Western-backed rebels have also been fighting Isil in Syria but have regularly been defeated by the group, which in June declared an “Islamic caliphate” in the territory it controls.

Activists have accused the Syrian army of avoiding confrontations with Isil because it has weakened rival rebel groups also battling President Bashar Al Assad.