The United States is “watching very carefully” for Islamic State militants operating outside Sirte as the jihadist group faces defeat in its former North African stronghold, a senior U.S. Department of State official told.

Libyan forces have been battling Islamic State in Sirte for more than six months and have reduced the amount of ground held by its fighters to a small cluster of buildings near the city’s Mediterranean sea front.

Since Aug. 1 the forces, led by brigades from the western Libyan city of Misrata, have been backed by U.S. air strikes.