Some 2,800 people have been displaced by fighting between rival militias over Tripoli, the UN humanitarian co-ordinator in Libya said.
Maria do Valle Ribeiro said clashes have prevented emergency services from reaching casualties and civilians, and have damaged electricity lines.
She warned that the increased violence is worsening the situation for migrants held in the capital’s detention centres.
Fighting was under way at the international airport, some 24 kilometres (15 miles) from central Tripoli.
The self-styled Libyan National Army, led by Khalifa Hifter, began an offensive against the capital last week.
Rival militias that support the UN-backed government in Tripoli have vowed to recapture all the areas recently seized by Hifter’s forces.
The two sides reported that at least 41 people, including civilians, had been killed since Thursday.