Free Syrian Army fighters rest with their weapons while they gather in what they said was an operation to travel to the northern countryside of Aleppo to fight ISIS in Aleppo, Syria, September 3, 2015.

At least 47 fighters were killed in clashes between ISIS and rival Syrian rebels, a monitor said on Saturday, in an area where the United States and Turkey are planning to open a new front against ISIS militants.

The renewed fighting raged on Friday around the rebel-held town of Marea, 20 km (12 miles) from the Turkish border, the Britain-based Syrian Observatory for Human Rights said.

The area falls within a “safe zone” Turkey said last month it would set up in northern Syria to help keep ISIS at bay.

ISIS said late Friday it had waged a fresh assault on Marea, killing “dozens” of Syrian rebels fighting against it.

The group last week encircled Marea, taking several villages around the town, in a blow to rebels who are likely partners for Ankara and Washington in any ground campaign.

A loss of Marea would make it harder for Turkey and the United States to open a new front against ISIS.

Western-backed rebels have sent in reinforcements from other parts of Aleppo province to try to beat back the jihadists, according to a rebel fighter.

ISIS holds large swathes of territory across Syria and Iraq, and has advanced in other areas of Syria in recent months. It is fighting rival insurgents, the Syrian military and Kurdish regional forces alike in Syria’s four-year-old civil war.