More than 120,000 people are trapped inside the eastern Libyan city of Derna by heavy fighting between Islamists and forces commanded by General Khalifa Haftar.
A U.N. report Thursday said Derna has suffered through "unprecedented" violence over the past several weeks, including bombardments and airstrikes.
There are severe shortages of food, water, medicine and electricity.
One report said there have been no deliveries of aid since mid-March.
Haftar's Libyan National Army is trying to take back the city from a coalition of Islamists and local fighters, whom he calls terrorists.
Derna is the only city in eastern Libya that the LNA does not control.