The jihadist group has grown bolder in recent weeks, setting up temporary checkpoints, attacking local forces, and taking over a village mosque 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 US air campaign. Most militants were killed in the nearly seven-month battle, but an unknown number fled into the desert to the south and west.
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 Taqrift, between Sirte and the town of Zillah, 306 km (190 miles) to the south east. The area is close to oil fields previously damaged by Islamic State attacks.
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 mounting frequent patrols to monitor Islamic State movements in areas under their control.
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.
Checkpoints
Both the main loose alliances in Libya have accused each other of allowing Islamic State space to operate in order to advance their own military ends.
Misratan forces, which aligned themselves with the UN-backed government in Tripoli, have complained of receiving little support as they try to prevent the jihadists regrouping.
Islamic State militants have stepped up their presence in several settlements east of Sirte, said Ibrahim Mlaitan, head of security for Sirte municipality. That included entering a local mosque to preach a sermon last Friday, he said.
"They set up checkpoints that last just for 10 minutes and then leave. They have been moving around freely in and out of these towns, and in the desert too," Mlaitan said.
Eastern military officials denied reports that Islamic State had taken control of one of the villages, Um Qandeel, but a military source acknowledged that the militants had set up checkpoints there during fleeting night visits.
"The terrorist organisation Daesh (Islamic State) from time to time tries to enter the coastal areas from the desert regions and they carry out kidnappings of civilians, and we are conducting surveillance on them," said LNA spokesman Ahmed al-Mismari.
Libya forces allied with the UN-backed government patrol to prevent Islamic State resurgence on the outskirts of Sirte, Libya, August 4, 2017
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.
