The head of Libya's unity government on Sunday urged his countrymen to close ranks to overcome political divisions in a country wracked by the threat of jihadists.

Prime minister-designate Fayez al-Sarraj spoke to reporters after a day of UN-hosted talks focused on creating a "unified" army in the oil-rich North African country.

The United Nations brokered late last year a power-sharing deal to form a Government of National Accord (GNA) but the Sarraj government is still struggling to assert its authority.

"We call again on all the children of the country who have not yet joined us to place the nation's interests before any other consideration," Sarraj said in Tunis.

On Saturday, the head of the UN's support mission in Libya, Martin Kobler, said that "all Libya's problems today are tied up to the security issue".

Speaking after the talks in the Tunisian capital, Kobler said the goal of the meeting was to create "a unified Libyan army under the command of the presidential council".

"Libya cannot be united as long as it has several armies," he said, according to an Arabic translation of his remarks.

The meeting also touched on humanitarian issues.

The GNA faces a fearsome set of military, economic and political challenges in a country where rival militias have vied for power since the overthrow of veteran dictator Moamer Kadhafi in 2011.

Forces loyal to the unity government are fighting to drive the Islamic State group out of the jihadists' coastal stronghold of Sirte.

The two-month battle has killed around 250 pro-GNA fighters and wounded more than 1,400, according to medical sources at the unity forces' command centre.

The GNA arrived in the capital three months ago, but it has not been endorsed by Libya's elected parliament, while a rival political authority based in the country's far east has refused to cede power.

A controversial general, Khalifa Haftar, controls forces loyal to this authority.

"I want to meet General Haftar, to see and understand his position. I contact him every week to arrange a meeting, but so far he has refused," Kobler said.

Libya's UN-backed presidential council would meet on Monday and Tuesday with "influential security parties" for further talks on a unified army, he said, without specifying who these parties would be.