Forces aligned with Libya’s U.N.-backed government suffered losses from snipers and mines on Friday as they battled Islamic State to win control of a strategic conference center in the coastal city of Sirte.

Islamic State is clinging on in the center of Sirte in the face of a two-month campaign by brigades mainly composed of fighters from the western city of Misrata. Their progress has been slowed by resistance from militants holed up in a 5 km area in central Sirte.

Losing Sirte would represent a major setback for Islamic State, which established its most important base outside Syria and Iraq in the Libyan city.

Sirte had been under the militant group’s control since last year, and it had extended its presence along about 250 km (155 miles) of coastline.