“International law requires that the authorities conduct prompt, effective and transparent investigations,” it said in a statement.
The Government of National Accord (GNA), which is recognised by the United Nations, retook control of Tarhouna on Friday after the eastern-based Libyan National Army (LNA) of Khalifa Haftar pulled out.
The GNA has said the Justice Ministry set up 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 stabilised 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.
(Reporting By Angus McDowall Editing by Marguerita Choy)