The UN mission in Libya has welcomed a second exchange of prisoners between the country's rival administrations and called for faster progress on other parts of an October truce.

The formerly warring sides exchanged a total of 35 prisoners in the small southwestern town of Shwairif under the auspices of a Joint Military Commission (JMC) set up under the hard-won ceasefire, the UN Support Mission in Libya (UNSMIL) said late Tuesday.

The two sides completed a first prisoner swap in the same town late last month.

"This second official exchange of detainees is the result of the steadfast commitment by the JMC to move forward with the full implementation of the ceasefire agreement," the UN mission said.

"In this spirit, UNSMIL calls on both parties to swiftly finalise ongoing negotiations to reopen the coastal road."

The main highway along Libya's Mediterranean coast linking the capital Tripoli in the west with second city Benghazi in the east has long been severed by the front line between the opposing administrations.

The UN-recognised Government of National Accord (GNA) controls Tripoli and most of the west while a rival administration dominated by military strongman Khalifa Haftar controls Benghazi and the east.

The fragile ceasefire between the two sides has largely held despite a threat by Haftar last month to resume fighting.

The next major test of the truce is a January 23 deadline for all foreign troops and mercenaries to leave Libya.

Turkey has deployed troops and Syrian militia allies in support of the GNA.

Russia, Egypt and the United Arab Emirates have dispatched military advisers and private contractors in support of Haftar.

UN chief Antonio Guterres proposed last month that international monitors be deployed to support the ceasefire amid hopes Libya can finally turn the page on a decade of war since a NATO-backed uprising ousted and killed longtime dictator Moamer Kadhafi in 2011.