Forces in eastern Libya loyal to the militia leader, Gen Khalifa Haftar, have been advancing westward, prompting a security alert in the capital, Tripoli.
The authorities there – who are recogized by the UN – have ordered a general mobilisation.
The escalation coincides with the arrival of the UN secretary-general, Antonio Guterres, who’s been seeking a truce between the rivals.
A spokesman for the eastern forces, Ahmad Mismari, said his troops were engaged in clashes with a rival militia based to the south of Tripoli.
He did not make clear whether there were plans to advance on the capital itself.
Rival Libyan factions have been fighting each other since the overthrow of the long-time leader Muammar Gaddafi in 2011.