The United Nations rights office called on Wednesday for an independent investigation into the discovery of mass graves at detention centres in Libya's capital Tripoli.

The Office of the High Commissioner for Human Rights said it was "shocked" by gross human rights violations uncovered at official and unofficial detention facilities run by the Stability Support Apparatus, a security institution.

It was established to uphold the rule of law and falls under the Presidential Council that came to power in 2021 with the Government of National Unity of Abdulhamid Dbeibah through a UN-backed process.

But Libya, a major oil producer in the Mediterranean, has known little law and order since a 2011 NATO-backed uprising that toppled ruler Muammar Gaddafi and eventually divided the country between warring eastern and western factions.

'Transparent investigations'

The OHCHR said the discovery of dozens of bodies and suspected instruments of torture and abuse confirmed longstanding findings by the UN that human rights violations were committed at such sites.

"We call on the Libyan authorities to conduct independent, impartial and transparent investigations into these discoveries," OHCHR said in a statement. It urged the authorities to preserve evidence and grant Libya's forensic teams, as well as the United Nations, full access to the sites.

Outright war fighting in Libya abated with a ceasefire in 2020 but efforts to end the political crisis have failed, with major factions occasionally joining forces in armed clashes and competing for control over Libya's substantial energy resources.

Armed clashes erupted on Monday evening and gunfire echoed in the centre and other parts of Tripoli following reports that the commander of one of its most powerful armed groups had been killed, three residents told Reuters by phone.