East Libyan forces said they had launched air strikes on Islamic State fighters after the militants made incursions south and east of their former coastal stronghold of Sirte.
The militant group has grown bolder in recent weeks, setting up temporary checkpoints, attacking local forces, and taking over village mosques to lead prayers during the Muslim holiday of Eid al-Adha, Libyan officials say.
The increased activity has raised concern that Islamic State could regroup around Sirte, from where it was driven out in December by local forces and a U.S. air campaign. Most militants were killed in the nearly seven-month battle, but an unknown number fled into the desert.
Sirte lies at the centre of Libya's Mediterranean coastline, on the dividing line between regions controlled by rival Libyan factions.
Forces loyal to eastern-based commander Khalifa Haftar said they had carried out air strikes on Sunday against militants in the area of Ain Tarqft, between Sirte and the town of Waddan, 230 km (140 miles) to the south.
Both Haftar's Libyan National Army (LNA) and forces from the port city of Misrata, which led the campaign in Sirte last year, say they are carrying out frequent patrols to monitor Islamic State movements in the area.
The LNA and Misratan brigades have been on opposite sides of a conflict that developed after the NATO-backed uprising that toppled Libyan strongman Muammar Gaddafi in 2011.
Islamic State exploited the turmoil to establish a foothold in Libya, taking complete control of Sirte in 2015 and using it as a base for hundreds of foreign fighters.