The head of Libya's unity government has asked for an official explanation from Paris about the presence of French troops in the conflict-ridden country, a statement said on Tuesday.

Fayez al-Sarraj told French ambassador Antoine Sivan on Monday that their presence in the country's east was "a violation of international customs", his government said on its Facebook page.

Sarraj asked for an explanation from the French government at a meeting in Mauritania where the prime minister-designate was attending an Arab summit, the statement said.

His comments came after France's President Francois Hollande confirmed last week that his country had soldiers in Libya, after three French troops were killed there during a mission to gather intelligence.

Rival militias have been vying for power in the oil-rich country since the overthrow of longtime dictator Moamer Kadhafi in 2011.

The Government of National Accord (GNA) headed by Sarraj, which is the result of a UN-brokered power-sharing agreement in December, has been striving to assert its authority amid the chaos.

France recognises the GNA, but also supports forces that are loyal to a parliament and government based in the country's east which are refusing to recognise and cede power to the UN-backed administration.

These forces led by General Khalifa Haftar have for the past two years been fighting armed groups including the Islamic State jihadist organisation in the city of Benghazi, located 1,000 kilometres (600 miles) east of Tripoli.

Forces loyal to the GNA have meanwhile been fighting to retake the city of Sirte from IS, after the jihadists overran the city -- 450 kilometres (280 miles) east of the capital -- in June last year.