United Nations' Secretary-General Antonio Guterres expressed deep shock late Friday at the discovery of mass graves in Libyan territory recently recaptured from forces commanded by Khalifa Haftar, and called for a transparent investigation.
The U.N. chief also called on Libya's U.N.-supported government to secure the mass graves, identify the victims, establish the causes of death and return the bodies to next of kin.
He offered U.N. support in carrying it out, U.N. spokesman Stephane Dujarric said.
"The secretary-general once again reminds all parties to the conflict in Libya of their obligations under international humanitarian law and international human rights law," Dujarric said.
The United Nations said earlier Friday that at least eight mass graves have been discovered, mostly in the town of Tarhuna, a key western town that served as a main stronghold for Haftar's east-based forces in their 14-month campaign to capture the capital Tripoli.
The discoveries have raised fears about the extent of human rights violations in territories controlled by Haftar's forces, given the difficulties of documentation in an active war zone.
Philippe Nassif, Amnesty International's director for the Middle East and North Africa, said the group was working to verify the mass killings.
"We want to be able to go in, or have the U.N. go in, and collect evidence of potential war crimes and other atrocities ... so eventually a process takes place where justice can be served," he said.
Last week, militias allied with the U.N.-supported government in Tripoli recaptured Tarhuna, some 65 kilometers (41 miles) southeast of the Libyan capital, their latest in a string of battlefield successes that reversed most of Haftar's gains.
Earlier, the government said it regained control of all of Tripoli's entrance and exit points and Tripoli airport.
Fathi Bashagha, the interior minister in the U.N.-supported government, said earlier this week that authorities were documenting evidence of alleged war crimes in Tarhuna, noting that preliminary reports indicated dozens of victims found in the city's mass graves had been buried alive.
Bashagha also said that special investigative teams uncovered a shipping container in Tarhuna full of charred bodies, presumably of detainees, and blamed powerful militias loyal to Haftar for "heinous crimes."
A feared Haftar-allied militia called al-Kaniyat, notorious for its targeting of dissenters, had controlled the town.
U.S. Assistant Secretary of State for Near Eastern Affairs David Schenker told reporters on Thursday he was "troubled" by reports that Tripoli forces had discovered bodies of civilians, in addition to land mines and other explosive devices in territory retaken from Haftar's forces.
Libya has been in turmoil since 2011 when a civil war toppled long-time dictator Moammar Gadhafi, who was later killed.
The oil-rich country has since split between rival administrations in the east and the west, each backed by armed groups and foreign governments.
Haftar's offensive is supported by France, Russia, Jordan, the United Arab Emirates and other key Arab countries. The government in Tripoli is backed by Turkey — which sent troops and mercenaries to protect the capital in January — as well as Italy and Qatar.
Guterres hopes that a cease-fire will be agreed soon, Dujarric said.
The intensified fighting has forced nearly 24,000 people to flee their homes in the last week, according to U.N. humanitarian officials.