Libyan militia fighters with anti aircraft guns and mortars fanned out across Tripoli's airport, transformed into a battlefield by two days of fighting that has cut the Libyan capital off from the outside world.

US Secretary of State John Kerry said 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 dictator Muammar Gaddafi.

At least 15 people have been killed in the capital and the eastern city of Benghazi since Sunday. Fighting between rival militias at the capital's airport damaged the control tower and wrecked 11 civilian aircraft parked on the tarmac. The main terminal building has been turned into a field hospital.

In Benghazi, irregular forces loyal to renegade former general Khalifa Haftar, a former Gaddafi ally, bombarded Islamist militia bases as part of his self-declared campaign to oust militants. Special forces clashed with militia fighters in the city.

The Tripoli airport battle has prompted the UN to evacuate its staff from the country, where a new government is struggling to impose order over streets prowled by fighters 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 Islamist-leaning militias 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. But months of protests at oil fields and ports caused government revenues to collapse last year, leaving the economy in tatters.

Nato helped rebels topple Gaddafi with air strikes, but the Western military alliance has not intervened to stem the subsequent chaos. Kerry said the United States, 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 international forces to improve security. But it is not clear whether there has been any real Libyan proposal, much less international willingness to send troops.

Western powers fear chaos in Libya will allow arms and militants to flow across its borders. The south of the vast desert country has become a haven for Islamist militants kicked out of Mali by French forces earlier this year.

ALLIANCES

A fragile government and parliament have been deadlocked in political struggles between rival Islamist, 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, an Islamist political bloc.

In a possible confrontation, the Misrata-based Central Shield Brigade called on all units to move to Tripoli within 24 hours to "secure state institutions ... and also due to a lack of ceasefire from armed forces inside Tripoli airport," a spokesman said, referring to Zintani troops located at the airport.

At least seven people have been killed since Sunday at Tripoli's airport, the worst fighting in the capital for six months. 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 to import replacement equipment 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.

Seeking to reconnect the country to international air traffic, the transport ministry said in statement late on Monday that it had opened the air space for Tripoli's smaller second airport, Mitiga, and the airport of Misrata to receive domestic and international traffic. It gave no details.

Security and medical sources said at least six people had been killed and 25 wounded in Benghazi in heavy fighting between security forces and rival militias since late Sunday.

In one bit of good news for the government, the authorities have managed to end a port blockade by a brigade of militiamen who had taken control of four main oil terminals to demand more autonomy for their eastern region. That protest and others at oilfields slashed the Opec country's production.-Reuters