Civilians caught in crossfire of IS-rebel clashes in northeast Syria

Heavy fighting between Islamic State and rebels gripped a town in northeastern Syria Saturday, activists said, after a lightning assault by the IS fighters cut a main supply route.

"Heavy clashes took place overnight between IS fighters and rebels inside the walls of Marea town," British-based monitor the Syrian Observatory for Human Rights said Saturday.

Up to 50 members of IS had been either killed or taken prisoner by Saturday afternoon, representatives of the Free Syrian Army told Sky News Arabia.

IS swept through rebel-held territory early Friday in a shock offensive in Aleppo province, cutting off the main road between Marea and Azaz, 20 kilometres to the northeast near the Turkish border.

The surprise advance came as the militants faced an offensive by a US-backed Kurdish-Arab alliance in its own heartland of Raqqa province further east.

Maamoun Khateeb, a journalist and activist from Azaz, told AFP that IS attacked Marea early Saturday, mainly from the east and north using tanks and two car bombs.

On Saturday there were reports of a third car bomb on the outskirts of Marea.

This advance has left around 15,000 residents remaining under siege inside Marea, he said.

The UN warned Saturday that civilians have been caught in the crossfire during intense fighting in the town.

Human rights groups have warned that the IS advance in Aleppo has left tens of thousands of displaced Syrians trapped along the closed Turkish border.

To the east in Raqqa province, warplanes from the US-led coalition conducted air strikes on IS positions north of Raqqa city, Observatory chief Rami Abdel Rahman said.

But, he said, the Kurdish-Arab alliance fighting IS north of its de facto Syrian capital had as yet made no strategic progress on the ground.