A car bomb at a checkpoint killed four troops from Libyan commander Khalifa Haftar's forces on Thursday, in an attack claimed by Islamic State, officials and security sources said.
Haftar's Libyan National Army is one of the most powerful armed brigades in Libya, where rival factions and their military backers have competed for control since the fall of former leader Muammar Gaddafi in a 2011 uprising.
"The car bomb explosion that targeted a checkpoint in Nawfiliya town has resulted in two killed of Sirte Security Directorate and some other wounded," LNA spokesman Ahmad Mesmari told Reuters.
Two security sources later said two more soldiers had died.
Islamic State claimed the attack, according to the militant group's AMAQ news agency.
Haftar's forces have been fighting against Islamist militants and other foes in eastern Benghazi and are besieging the city of Derna to try to oust militants there.
Thursday attack in Nawfiliya was 80 km (50 miles) from Ras Lanuf, part of Libya's "Oil Crescent" and one the OPEC country's major oil exporting terminals.
Libyan officials are concerned Islamic State may try to regroup there after defeat in nearby Sirte city last year.