Forces aligned with a UN-backed government in Tripoli said on Monday that three of their men had been killed in air raids against a desert air base by rivals allied with eastern-based commander Khalifa Haftar.
The oil-producing North African state slipped into turmoil during the 2011 uprising that ended Muammar Gaddafi's 42-year-old and has been riven by factional strife since then.
The struggle for control around Tamanhent air base 30 km (19 miles) northeast of Sabha risks escalating into the first major confrontation between forces officially linked to the Government of National Accord (GNA) and Haftar's Libyan National Army (LNA).
Haftar is aligned with an eastern parliament and government that have spurned the GNA since it arrived in the capital Tripoli, in the far west of the country, a year ago.
His forces have been extending their reach along Libya's central Mediterranean coastline and into the desert regions of Jufra and Sabha, and say they also intend to take control of Tripoli.
After an LNA strike against Tamanhent last week the GNA warned of the risk of civil war and said it was mobilising forces to repel the attack.
Tamanhent is controlled by a force from Misrata, a militarily powerful western city that has backed the GNA. Air strikes on Monday killed three men stationed there and wounded at least one more, according to Mohamed al-Gasri, a spokesman for the defence ministry in Tripoli.
An eastern military source said there had also been ground clashes around Samnu, about 50 km (30 miles) northeast of Sabha. A medical source in Sabha said three LNA troops had been killed.
Since 2014, loose and shifting military alliances based in the east and west of Libya have been engaged in a stop-start conflict which the GNA has been unable to resolve.