At least 47 fighters were killed in clashes between the Islamic State group 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 IS militants.
The renewed fighting raged on Friday around the rebel-held town of Marea, a "safe zone" 20km (12 miles) from the Turkish border, the Britain-based Syrian Observatory for Human Rights said.
Turkey had said last month it would set up a new front in northern Syria to help keep Islamic State at bay.
The extremist group, however, said on Friday that it had waged a fresh assault on Marea, killing "dozens" of Syrian rebels fighting against it.
IS had last week encircled Marea, taking several villages around the town, in a blow to rebels who are likely partners to 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 Islamic State.
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.
Islamic State 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.