The United Nations says talks to form a unity government in Libya have restarted in Morocco but without the Tripoli delegation, which is reorganizing its negotating team.

Thursday's statement said talks had begun but this round would be without the Tripoli-based negotiators, whose chief resigned following differences with the faction's leader.

The Tripoli delegation is the only one that has not initialled the national unity government agreement, a product of seven months of negotiations.

UN represenative Bernadino Leon warned in the statement that time was running out as the country faces economic and political challenges.

The country is controlled by competing militias and the Islamic State group has a strong presence.