Updated

Activists say fighters from the extremist Islamic State group stormed parts of a besieged army base in northern Syria, setting off clashes that left dozens killed or wounded on both sides.

Thursday's attack came a week after Islamic State fighters captured a gas field in the central province of Homs, an attack that left more than 200 people dead.

The Britain-based Syrian Observatory for Human Rights says the assault began with two suicide car bomb attacks.

It added that Syrian army helicopters attacked jihadi positions around the base, known as Division 17, in the province of Raqqa.

The Local Coordination Committees, another activist group, says the air force conducted 12 raids around Division 17 and the nearby provincial capital of Raqqa, which is controlled by the Islamic State group.