The UN expressed "horror" after at least eight mass graves were discovered in an area retaken by Libya's internationally recognised government from renegade military commander Khalifa Haftar's forces.
According to the Tripoli-based Government of National Accord (GNA), most of the graves were found in Tarhuna, Haftar's last stronghold in western Libya. The city was used by his forces as a launchpad during an ill-fated 14-month offensive to seize the capital from the GNA.
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The United Nations Support Mission in Libya (UNSMIL) "notes with horror reports on the discovery of at least eight mass graves in past days, the majority of them in Tarhuna," it wrote on Twitter.
"International law requires that the authorities conduct prompt, effective & transparent investigations into all alleged cases of unlawful deaths."
Reporting from Misrata, Al Jazeera's Malik Traina said the GNA, which seized Tarhuna on June 5, reported recovering "over a hundred bodies in these mass graves".
The GNA said the remains were those of captured GNA soldiers as well as civilians, Traina said.
"These mass graves are yet another indication of the brutality of the Libyan conflict and the toll on the residents in the area," he added.
On Thursday, the GNA's justice ministry launched a committee to investigate the graves, according to the UN mission.
UNSMIL called on members of the committee "to promptly undertake the work aimed at securing the mass graves, identifying the victims, establishing causes of death & returning the bodies to next of kin".
The United States also expressed concern over the grim findings.
"We are troubled by reports that GNA forces are discovering bodies of civilians, IEDs and land mines in areas retaken from the LNA," said David Schenker, US assistant secretary for Near Eastern Affairs.
"We are similarly concerned that a GNA offensive on Sirte would have serious humanitarian consequence. When armed groups and their external backers escalate, the Libyan people suffer."
Reports of atrocities
In March, UNSMIL said it received reports of hundreds of enforced disappearances, torture, killings and displacement of entire families in Tarhuna by forces loyal to Haftar.
Among the victims were "private individuals, state officials, captured fighters and civil society activists", according to the mission.
UNSMIL said it also verified numerous summary executions at Tarhuna prison on September 13.
Toby Cadman, an international human rights lawyer, told Al Jazeera that based on the reported conditions of the dead, including reports some may have had their hands tied behind their backs, the newly discovered graves appeared to be evidence of war crimes.
"Of course, they will need to be an investigation to identify the cause of death," he said.
Human Rights Watch Libya senior researcher Hanan Salah said the GNA should invite neutral international forensic experts to help preserve possible evidence of crimes and identify the remains.
"We urge the GNA to follow through with their promise to investigate apparent mass graves in a speedy and transparent manner," she said.
Libya, a major oil producer, has been mired in turmoil since 2011, when longtime ruler Muammar Gaddafi was toppled in a NATO-backed uprising.
It is now split between two rival administrations: the GNA in Tripoli and the eastern-based House of Representatives allied with Haftar.
The GNA is backed by Turkey while Haftar's Libyan National Army is supported by the United Arab Emirates, Egypt and Russia.
In recent weeks, the GNA, with the support of Turkey, has made major military gains, forcing Haftar's forces to retreat.
The GNA has since launched a military operation to take the central coastal city of Sirte and al-Jufra further south.
