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.
Update: Medecins Sans Frontieres have said they are "extremely concerned" by the fighting in Tripoli.
"MSF is extremely concerned for civilians caught in the ongoing fighting in Tripoli, including those refugees and migrants currently trapped in detention centres in or near the affected areas.," Craig Kenzie, Project Coordinator for MSF's Operations in Tripoli.
"The fighting has made refugees and migrants in detention exceptionally vulnerable and severely reduced the capacity of the humanitarian community to provide timely lifesaving response and urgently needed evacuations."
"This is the third time in the past seven months that Tripoli has erupted in conflict, yet many people trapped in detention are there because of European member state policies that enable the Libyan Coast Guard to intercept people at sea and forcibly return them to Libya in violation of international law," Mr Kenzie added.
- Press Association; Additional reporting by Joel Slattery