The Syrian Observatory for Human Rights reports that Islamic State militants are advancing against rival fighters in northern Syria.
The Islamic State is reportedly claiming areas near a border crossing with Turkey, which could be a major victory for the group. It would cut off supply routes for the northern rebel alliance, which was formed last December and includes both Western-backed fighters and Islamist militants.
The loss of the territory could make it difficult for for the alliance to bring weapons into Aleppo. Syria's largest city and scene of some of the worst fighting during the four-year conflict.
ISIL has recently seized swathes of territory across Syria and Iraq. It has captured the central Syrian city of Palmyra and the last border crossing between Syria and Iraq in the east.