Fighters from Libya's Zintan militia, armed with anti-aircraft guns and mortars, have fanned out across Tripoli's airport, transformed into a battlefield by two days of fighting that has cut the capital off from the outside world.
Fighting between rival militias has left the control tower damaged and wrecked 11 civilian planes parked on the tarmac. The main terminal building has been turned into a field hospital.
At least 15 people have been killed in the capital and the eastern city of Benghazi since Sunday, the Reuters news agency reported.
In Benghazi, irregular forces loyal to renegade former general Khalifa Haftar, a former Gaddafi ally, bombarded rival armed group bases as part of his self-declared campaign to oust militias.
Special forces clashed with militia fighters in the city.
US Secretary of State John Kerry said on Tuesday that Washington was working to help end to violence that has brought the north African country to the edge of chaos three years after the uprising that toppled Muammar Gaddafi.
The airport battle has prompted the UN to evacuate its staff from the country, where a new government is struggling to impose order over militias who rose up against Gaddafi and never disarmed.
The airport area is under the control of former fighters from the western town of Zintan who have held it since the fall of Tripoli in 2011.
Rival militias, who want to impose Islamic law, have fought with the Zintanis in recent days but failed to dislodge them.
For the past three years, the new authorities have tried to rein in militia fighters by putting them on the government payroll.
NATO helped rebels topple Gaddafi with air strikes, but the Western military alliance has not intervened since then to stem the subsequent chaos.
Kerry said the US, whose ambassador was killed in an attack in Benghazi in 2012, had sent diplomats to seek consensus among Libyan political groups.
"We are deeply concerned about the level of violence in Libya," Kerry told a news conference in Vienna.
"It is dangerous and it must stop. We are working very, very hard through our special envoys to find the political cohesion... that can bring people together to create stronger capacity in the government of Libya so that this violence can end."
Government spokesman Ahmed Lamine said Tripoli was studying the possibility of bringing in international forces to improve security.
Western powers fear chaos in Libya will allow arms and fighters to flow across its borders.
A fragile government and parliament have been deadlocked in political struggles between rival religious, nationalist and tribal factions, each allied to competing brigades of heavily armed former rebels who refuse to disarm.
The Zintanis and their allies in Tripoli are loosely aligned with the more nationalist National Forces Alliance led by a former Gaddafi official.
Their main rivals include brigades from the city of Misrata and other militias closer to the Justice and Construction Party, a religious political bloc.
The UN mission in Libya said the closure of Tripoli airport and the deteriorating security situation made it impossible for it to operate.
A Zintani airport official said the control tower would need replacement equipment to be imported before it could be fixed.
The airport in Misrata was also forced shut on Monday, while in the east, Benghazi airport has been closed since May.