At least five people were killed in fierce clashes on Friday between rival armed groups in the Libyan capital Tripoli, seat of a UN-backed unity government, a hospital official said.
The city has been gripped by a power struggle between dozens of militias since a NATO-backed uprising toppled longtime dictator Moamer Kadhafi in 2011.
An official at a Tripoli hospital gave a "provisional" toll of five dead and several wounded, including civilians.
AFP journalists heard explosions and artillery fire as fighting broke out in the Abu Slim, Al-Hadhba and Salaheddin districts in the south of the city.
A militia loyal to the unity government, the Abu Slim Deterrence Force, said on its Facebook page that it lost five fighters but it was unclear if they were included in the hospital toll.
UN envoy Martin Kobler issued an appeal for a halt to the fighting.
"Voices of reason should prevail for the benefit of the country," he said. "Political aims must not be pursued through violence. Civilians must be protected."
Witnesses said tanks had been deployed in the fighting.
British ambassador Peter Millett tweeted that he could hear explosions and artillery in south Tripoli.
He condemned "action by these militias who threaten security" in the run-up to the Muslim holy month of Ramadan, which begins on Saturday in Libya.
Groups hostile to the Government of National Accord (GNA) said they had attacked loyalist forces.
The fighting started around a complex of luxury villas that until March served as the headquarters of militias loyal to former prime minister Khalifa Ghweil.
Ghweil was ousted from power when the GNA took office in March 2016, and has refused to recognise the new administration.
Loyalist forces seized the villas in four days of intense fighting in March that saw them expand their control over the capital.
Tripoli had been relatively calm since, but dozens of armed groups still operate -- including several that support Ghweil.
The GNA has won the support of various militias since it took office in March last year, but several parts of Tripoli remain beyond its control.
Relying on militia support and pitted against a rival administration in the east, the GNA has struggled to assert its authority.