The United Nations Support Mission in Libya called Sunday for an investigation into a political activist's death while detained at an eastern military base controlled by military strongman Khalifa Haftar.

UNSMIL also demanded the "immediate release" of other prisoners it said were being detained "arbitrarily" by the war-torn country's eastern-based authorities.

In a statement on X, the UN mission said it was "deeply saddened by the death of activist Siraj Dughman while in custody at Rajma military camp" and urged the Libyan "authorities to conduct a transparent and independent investigation into the circumstances surrounding his death".

Plagued by political instability and violence since the overthrow of longtime dictator Moamer Kadhafi in 2011, Libya is split between an internationally recognized government, based in Tripoli, and a rival administration in the east backed by Haftar.

The base at Rajma, about 25 kilometers (16 miles) east of Benghazi, serves as Haftar's headquarters.

In a video published on Saturday, the Haftar-affiliated Eastern Internal Security Agency confirmed Dughman's death.

The agency said it had commissioned a forensic examination according to which Dughman had died "while attempting to escape the prison on Friday" when he fell "from a window, fracturing his skull".

The agency said he was arrested in October 2023 together with several others accused of "participating in a campaign" inciting the "overthrow of official state agencies" including Haftar's forces.

UNSMIL said that Dughman "was arbitrarily arrested and detained in 2023" with other Benghazi-based staff members of the Libyan Center for Future Studies, an independent think tank, who "was never formally charged or appeared in court".

Dughman was the director of the organization's office in Benghazi, eastern Libya's main city.

Extrajudicial arrests, detentions, and assassinations of political dissidents, activists, and human rights defenders have become common in Libya, particularly in the North African country's east.

The Libyan Center for Future Studies said the security agency was "responsible for his death" which occurred in "obscure circumstances".