Two guards loyal to Khalifa Haftar’s Libyan National Army were killed in an attack Saturday claimed by ISIS, an LNA source said.
Four guards were also kidnapped in the attack on a checkpoint in the central region of Zella, the same source said.
ISIS said in a brief statement that it had targeted “Haftar’s militias”.
The media arm of Haftar’s LNA said in a statement a “terrorist attack” had been repelled, but did not give any details.
The raid in Zella, close to an oilfield some 800 kilometers (500 miles) southeast of the capital Tripoli, is the third ISIS attack targeting Haftar's forces in recent weeks.
At least nine people were killed in a May 4 attack in the southern city of Sebha, then five days later gunmen killed two civilians in Ghodwa in southern Libya.
The LNA launched an operation in January to purge southern Libya “of terrorist groups and criminals”.
After securing support from local tribes, it seized several towns.
Haftar then launched an offensive on April 4 against Tripoli, the seat of the internationally recognized Government of National Accord (GNA).
The LNA made initial gains but pro-GNA forces launched a counter-offensive, resulting in a stalemate on the southern outskirts of the city.
The fighting around Tripoli between Libya’s two main factions has created fears that ISIS could re-emerge in the country.
The extremists were pushed out of their stronghold Sirte by forces loyal to the GNA in December 2016.
But despite losing its main territory, east of Tripoli, ISIS has repeatedly shown itself capable of launching deadly attacks.
Libya has been mired in chaos since the NATO-backed uprising that deposed and killed dictator Muammar Gaddafi in 2011.