Fighting between rival forces over Libya's capital that began last week has displaced 2,800 people so far, the UN said as the death toll climbed to 49, including civilians.
The violence, triggered by a Libyan commander whose forces advanced on Tripoli and clashed with rival militias supporting the UN-backed government there, has threatened to ignite civil war on the scale of the 2011 uprising that toppled and killed longtime dictator Muammar Gaddafi.
The UN humanitarian coordinator in Libya, Maria do Valle Ribeiro, said the clashes have prevented emergency services from reaching casualties and civilians, and have damaged electricity lines.
The increased violence is also worsening the situation for migrants held in detention centres in the Libyan capital, she warned.
Fighting was under way on Monday at Tripoli's shuttered international airport, some 24 kilometres (15 miles) from central Tripoli.
Ahmed Musbah, a resident who lives near the area, said he could hear shooting coming from the direction of the town of Bin Ghashir, south of the airport.
"The sound of fighting seems to be closing in," he said.
The self-styled Libyan National Army, led by Khalifa Hifter, said on Saturday it seized the area.
However, the militias supporting the UN-backed government in Tripoli claimed on Monday they recaptured the facility.
The airport has not been functioning since fighting in 2014 destroyed much of it.
The Health Ministry of the Tripoli-based government said at least 27 people, including civilians, have been killed and at least 27 wounded since Thursday, when Hifter forces began their offensive against the capital.
The media office of Hifter's army said 22 of their troops had been killed since the offensive began.
The UN envoy for Libya, Ghassan Salame, said he met with Fayez Sarraj, head of the government in Tripoli, on Monday to discuss how the UN mission "can assist at this critical and difficult juncture".
Since Gaddafi's ousting, oil-rich Libya has been gripped by unrest, governed by rival authorities in the east and in Tripoli, in the west, each backed by various militias and armed groups fighting over resources and territory.
In Cairo, Agila Saleh, head of Libya's east-based parliament, voiced support for Hifter's offensive and the Libya National Army, saying that militias have been "hijacking" the capital.
"The Libyan army moved towards Tripoli with one goal, to free Tripoli from armed militias," he told reporters after his meeting with the Arab League's secretary-general Ahmed Aboul Gheit.
On Sunday, Marine Corps General Thomas Waldhauser, the head of US Africa Command, said the United States has temporarily withdrawn some of its forces from Libya due to "security conditions on the ground".
A small contingent of American troops has been in Libya in recent years, helping local forces combat Islamic State and al Qaida militants, as well as protecting diplomatic facilities.
