(CNN)At least eight mass graves been reportedly been discovered in Libya, according to the United Nations Support Mission in Libya.
In a tweet, the UNSMIL expressed "horror" over "reports on the discovery of at least eight mass graves in past days, the majority of them in Tarhuna."
Last week, Libya's UN-recognized Government of National Accord (GNA) announced control over Tarhuna, a key city to the southeast of capital Tripoli, after pushing back eastern forces led by renegade general Khalifa Haftar. The town was previously occupied by the self-styled Libya National Army (LNA) forces loyal to Haftar.
The city was also under the control of the Kaniyat militia, which is allied with Haftar. Kaniyat took control of the city last year when the LNA launched an offensive to take the capital. The militia has long been accused of committing atrocities in Tarhuna.
The UN-recognized Government of National Accord has promised to investigate the mass graves. "We discovered and are documenting many heinous crimes committed by the militias that controlled the city of Tarhuna," Minister of Interior Fathi Basghagha said in a statement on Thursday. "We found a container where all the detainees were burned. We also found several mass graves, according to our initial reports dozens were buried alive"
Tarhuna had been critical for Haftar's forces as a supply route and strategic base for their assaults on Tripoli. The town is the last major stronghold by Haftar's forces in western Libya.
The UNSMIL said the Libyan Justice Minister will establish a committee to look into the graves. "We call on its members to promptly undertake the work aimed at securing the mass graves, identifying the victims, establishing causes of death and returning the bodies to next of kin," the statement added.
On Thursday, the Justice Ministry announced that it had established a committee to investigate the mass graves and identity the victims and the circumstances of their deaths.
In response, the US Embassy in Libya tweeted, "The United States shares UNSMIL's horror and supports immediate efforts by Libyan authorities and international bodies to investigate these intolerable abuses and bring perpetrators to justice."
Assistant Secretary of State for Near Eastern Affairs, David Schenker, also responded, describing reports on the mass graves in Libya "truly disturbing."
The Libyan conflict has grown into a proxy war between foreign powers and fighters over the last year. While the UN-recognized GNA is backed by Turkey, the Benghazi-based eastern government is supported by the UAE, Egypt and Russia.
Human Rights Watch (HRW) has called for an international investigation into the killings. "For 15 months, Haftar-affiliated groups have been operating with impunity in Tarhuna and elsewhere, as foreign backers shielded them from scrutiny for abuses that apparently include unlawful killings, disappearances and torture," Hanan Salah, senior Libya researcher at HRW, told CNN.
"It is time for the United Nations Human Right Council to finally establish an international investigation into violations by all sides in Libya, which would send an unmistakable signal that those who commit or have command responsibility over war crimes, will eventually be held accountable," said Salah
Salah also called on the internationally-recognized government in Tripoli to ensure that evidence on the sites is protected from tampering and invite neutral international forensic experts to the mass graves. "Their families deserve to know," Salah added.