The UN mission said it was “concerned” by the deaths, which came after “allegations of ill-treatment and torture” since their arrest in the main eastern city of Benghazi on October 7.
Their arrest came after fighting broke out between loyalists of Barghathi, who was a minister in the UN-recognized government based in Tripoli, and forces affiliated with military strongman Khalifa Haftar, who controls the rival administration in the east.
The UN mission called on “competent Libyan authorities to conduct an independent and transparent investigation into these deaths and provide information on the fate of those still missing.”
“Given limited official information, the exact cause of these deaths remains unclear,” it added.
Authorities in the east have made no comment on the deaths.
It was not clear where in Benghazi Barghathi and his fellow detainees were held.
The former colonel had returned to the eastern city after years in exile, and the arrests came following a campaign by pro-Haftar media labelling him and his supporters a “cell of saboteurs.”
Since a NATO-backed uprising led to the overthrow and killing of longtime dictator Muammar Qaddafi in 2011, Libya has been beset by years of conflict involving a myriad of militias, jihadists and foreign mercenaries.
The country remains split between a UN-backed government based in the capital Tripoli run by Prime Minister Abdulhamid Dbeibah, and the rival eastern-based administration backed by Haftar.