"UNSMIL notes with horror reports on the discovery of at least eight mass graves in past days, the majority of them in Tarhuna," the UNSMIL said in a statement on Thursday.
"International law requires that the authorities conduct prompt, effective and transparent investigations into all alleged cases of unlawful deaths."
The UN-backed government accused the rival eastern-based army of committing war crimes in Tarhuna, the city that used to be the main operation centre for the eastern-based army that is located some 90 km south of the capital Tripoli, reports Xinhua news agency.
On Wednesday, Interior Minister of the UN-backed government Fathi Bashagha had confirmed the discovery of the mass graves in Tarhuna.
But he did not specify the exact number of graves.
There was no comment on the development from the east-based army, which has been retreating in recent weeks in its military campaign against the troops of the UN-backed government.
Also on Wednesday, the warring sides in the country began engaging in a new round of ceasefire talks, after heavy fighting around the central city of Sirte.
The east-based army has been fighting the UN-backed government troops for more than a year aiming to take control of Tripoli and topple the UN-backed government.
The fighting has killed and injured hundreds of civilians and displaced more than 150,000 others.
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