BEIRUT - Islamic State 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 40 km (25 miles) east of the city of Raqqa, is the Syrian army's last foothold in an area otherwise controlled by the Islamic State group that has seized large areas of Syria and Iraq in recent months.

There were fierce clashes between Islamic State fighters 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 Islamic State fighters 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.

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