Libya's UN-backed unity government on Monday moved into its official Tripoli offices, more than 100 days after working from a naval base in the capital.

The move comes three months after Prime Minister Fayez al-Sarraj sailed into Tripoli under naval escort along with several members of his Government of National Accord (GNA).

"Today we were handed the (formal) government headquarters, after spending more than 100 days in the naval base," Deputy Prime Minister Mussa al-Koni told reporters.

He said this "is the real start" of the government's work.

Sarraj chaired a cabinet session at the official seat of government after a ceremony attended by police officials and some ministers.

The GNA was the result of a UN-brokered power-sharing deal struck in December, but it is still struggling to unite the divided North African country and fully assert its authority.

A rival government in the eastern city of Tobruk is refusing to cede power until Libya's elected parliament passes a repeatedly delayed vote of confidence.

However, the GNA has won the loyalty of the central bank and national oil corporation -- depositors of the country's wealth -- as well as cities and armed groups in western Libya.

Since March 30, when Sarraj arrived in Tripoli, the GNA has also taken control of several key ministries.

The GNA faces a fearsome set of military, economic and political challenges in a country that has faced chaos since the overthrow of veteran dictator Muammar Gaddafi in 2011.

Libya's oil-dependent economy has also been hit hard by the sharp fall in world prices.