Heavy fighting has been reported in northern Syria as so-called Islamic State tries to capture the rebel-held town of Marea, near the Turkish border.
IS fighters began a surprise offensive in the area on Thursday and attacked Marea early on Saturday with tanks and car bombs, monitors said.
The UN fears for tens of thousands of civilians trapped in the fighting.
The area between the city of Aleppo and the Turkish border is split between IS, rebel and government-controlled zones.
A nationwide, but often violated ceasefire, between rebel and government forces, brokered by the US and Russia in February, does not include jihadist groups such as the self-styled Islamic State.