Fighting between powerful militias battling for control of Tripoli's airport broke out again on Friday, just hours after they had agreed a truce, an airport official and witnesses said.
The clashes came as the government sought United Nations help to prevent the country from becoming a "failed state".
"The airport was once again today hit by mortar fire which struck the security offices," but caused no casualties, airport security official Al-Jilani Al-Dahech told AFP.
Airport security forces returned fire, he said without giving further details.
Libya's main international airport has been closed since Sunday, with rockets causing damage to aircraft and the main terminal building amid warnings by officials the facility could remain closed for months.
The violence erupted when Islamist gunmen from the city of Misrata attacked anti-Islamist fighters from the city of Zintan who have been controlling the airport for the past three years.
The rival fighters are among several heavily-armed militia groups who hold sway over Libya since they fought in the NATO-backed 2011 uprising that toppled dictator Moamer Kadhafi.
Friday's clashes broke out just hours after the mayor of Tripoli announced that the rival militias had agreed to observe a truce and that control of the airport would be handed over to neutral forces.
Gunfire and blasts were also heard in Abu Slim neighbourhood, just south of the airport, an AFP correspondent said while residents said the battles pitched rival militias against each other.
The renewed violence also came hours after Foreign Minister Mohamed Abdelaziz asked the United Nations for help to build up Libya's army and police to protect vital sites such as the airport.
Relentless violence across Libya in the past months has sparked fears of all-out civil war and Abdelaziz told the Security Council his country could become a "hub for attracting extremists".
The fighting also mirrors a deadly power struggle between liberals and Islamists in the North African country.
- 'Battle of revolutionaries' -
Mokhtar Lakhdar, a commander for the Zintan forces, told AFP that a truce had been agreed under the authority of the city's government council.
Ahmed Hadeia, a spokesman for Misrata fighters, said the ceasefire was "only around the airport" and did not include other sites controlled by Zintan forces.
Misrata leaders said Thursday that the fighting at the airport was a "battle of revolutionaries... against followers of the old regime" of Kadhafi.
The clashes revived fears of the conflict spreading inside Tripoli itself, with official results still awaited from a June 25 election to the parliament previously dominated by Islamists.
"Should Libya become a failed state, kidnapped by radical groups and warlords, the consequences would be far-reaching and perhaps beyond control," Abdelaziz told the Security Council.
Libya could become a "hub for attracting extremists", feeding radicalism and the arms flow in the region and further afield in Syria, he said.
"We are not asking for military intervention," he said. "We are asking for a team from the UN specialised in the field of security."
In a statement, the 15-member Security Council said it condemned the recent violence in Libya and said it made "it even more difficult for the Libyan authorities to govern effectively".
The Council would ask UN Secretary General Ban Ki-moon to examine the Libyan request for aid and present "options", said Eugene-Richard Gasana, the Rwandan chair of the UN body.
Last week, the United Nations evacuated its staff from Libya after the latest upsurge in fighting.
Abdelaziz said a new UN mission to help train the security forces would ensure Tripoli keeps control of vital oil revenue after militant groups seized oil terminals last year.
The blockades of the oil facilities, that finally came to an end this month, deprived Libya of more than $30 billion in revenue over 11 months, the foreign minister said.