TRIPOLI - Officials in Libya's UN-recognised government say they plan to confront Moscow over the alleged deployment of Russian mercenaries to fight alongside their opponents in the country's civil war.

Libyan and US officials accused Russia of deploying fighters through a private security contractor, the Wagner Group, to key battleground areas in Libya in recent months.

They say the Russian fighters are backing renegade military commander Khalifa Haftar, whose forces have been trying for months to capture the capital, Tripoli, where the UN-recognised Government of National Accord is based.

The GNA has documented between 600 and 800 Russian mercenaries in Libya and is collecting their names on a list to present to the Russian government, according to Khaled al-Meshri, head of the GNA's Supreme Council of State.

"We are going to visit Russia after we collect all evidence and present to the authorities and see what they say," al-Meshri told The Associated Press last week.

He did not say when the visit would take place. Moscow has repeatedly denied playing any role in Libya's war.

Haftar's Libyan National Army - made up of army units, ultra-conservative militias, and tribesmen - launched its offensive on Tripoli in April after seizing much of eastern Libya from rivals in recent years.

Haftar is backed by the United Arab Emirates and Egypt, as well as France and Russia, while the GNA receives aid from Turkey, Qatar and Italy.

Libya was plunged into chaos when a NATO-backed uprising toppled longtime ruler Muammar Gaddafi in 2011. The country is now split between a government in the east, allied with Haftar, and the GNA in Tripoli in the west. Both sides are bolstered by militias.

Fighting has stalled in recent weeks with both sides dug in and shelling one another along Tripoli's southern reaches.

David Schenker, US assistant secretary of state for near east affairs, told reporters last week the State Department is working with European partners to impose sanctions on the Russian military contractor responsible for sending fighters to Tripoli. (FA)