The United States Wednesday said it was "concerned" about reports that radical extremists with avowed ties to the Islamic State are destabilising eastern Libya.

"We are closely monitoring the situation and are concerned by the destabilizing threat that militias and terrorist groups pose to the Libyan people and government," said State Department spokesman Jeffrey Rathke.

"We have seen reports that some violent extremist factions have pledged allegiance to ISIL and sought to associate themselves with it," Rathke said, amid news reports that IS-linked militants have overrun the town of Derna in eastern Libya.

"We continue to watch for signs that these statements amount to something more than purely rhetorical support," he said.

Rathke also voiced US support for a truce reached Wednesday between combatants in Libya's second city Benghazi, where more than 350 people have been killed since the launch of a government-backed offensive against Islamist militias who took near total control of the city in July.

Libyan authorities have struggled to assert control over powerful militias which ousted longtime autocratic leader Moamer Kadhafi in a 2011 revolt.

Rathke said Washington strongly backed the truce.

"We welcome this announcement and the reports that the parties are adhering to the ceasefire," the US spokesman said.

"We call on all Libyans to support the ceasefire, to allow the Red Crescent to evacuate civilians from affected areas, and to allow affected civilians the opportunity to address their immediate humanitarian needs," he added.

"Libya's problems are political in nature and require a political solution," Rathke said.

The UN announcement said pro-government forces and Islamist militias fighting for control of Benghazi had agreed to a 12-hour humanitarian ceasefire.

The UN-brokered truce was the first in Benghazi since the launch of a government-backed offensive to recapture the eastern city from Islamist a month ago.

The move aims to allow the evacuation of civilians, the retrieval of bodies, the removal of sewage and the restocking of food and medical supplies, the UN mission in Libya said.