In a statement released on Saturday, UN spokesperson Stephane Dujarric said the secretary-general is calling for a thorough and transparent investigation on the discovery of eight mass graves in the town of Tarhouna after the internationally recognized government recaptured it from eastern-based forces.

"International law requires that the authorities conduct prompt, effective and transparent investigations," the spokesperson said in the statement.

In a tweet, the UN Support Mission in Libya (UNSML) expressed "horror" over the report of the mass graves.

The Government of National Accord (GNA), which is recognized by the United Nations, retook control of Tarhouna last week after the eastern-based Libyan National Army (LNA) of Khalifa Haftar pulled out.

The GNA has announced the Justice Ministry had established a committee to investigate the discovery of mass graves, but it has not yet said how many bodies have been found so far.

Tarhouna had been held by the LNA-aligned Kani family, which controls a powerful local armed group known as the Kaniyat. When it was recaptured, dozens of bodies were also found in the town morgue, the GNA said.

The town was captured during a sudden change in the military map that also drove the LNA from its last foothold in Tripoli and other northwestern towns 14 months after launching a campaign to capture the capital.

The front line has since stabilized during recent days near the central coastal city of Sirte.

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.

In response, the U.S. Embassy in Libya also announced that the U.S. shares horrors with UNSMIL and fully supports efforts by Libyan authorities and international bodies to investigate the "intolerable abuses and bring perpetrators to justice".

The Libyan conflicts has grown into a proxy war between foreign and fighters in the last years. The UN-recognized GNA is backed by Turkey while the Benghazi-based eastern government is supported by the UAE, Egypt and Russia.

(With input from agencies)