Bodies of children were among those found in the Libyan town of Tarhouna after eastern-based forces and their local allies withdrew this month, Red Crescent and Tripoli government officials said on Tuesday.
The evidence of what rights groups have called possible war crimes came as Libya's frontlines suddenly shifted, displacing thousands of civilians and leaving a trail of landmines hidden in residential areas.
The internationally recognised Government of National Accord (GNA) retook Tarhouna, southeast of Tripoli, on June 5 as an offensive by the eastern-based Libyan National Army (LNA) to capture the capital collapsed.
GNA forces found 106 bodies in the hospital and local people discovered eight mass graves around Tarhouna, prompting the United Nations to call for an urgent and open investigation.
Libyan Red Crescent representative Faisal Jalwal told a news conference that 29 of the bodies found in the hospital had been identified and that they included women and children.
Kamal Al-Siwi, head of the GNA's missing persons bureau, said about 10 bodies had been disinterred from one of the eight mass graves that had been found.
Tarhouna had for years been controlled by the local Kani family and its armed group, popularly known as the Kaniyat, which had been loyal to different sides during Libya's chaotic civil war.
The town served as an important stronghold for the LNA during its attack on Tripoli and the Kaniyat fought alongside the LNA in the southern suburbs of the capital.
A GNA justice ministry official, Nasser Ghaita, said it had issued 23 arrest warrants over the case and asked for UN technical help. The GNA said on Saturday that "entire families" had been killed.
The LNA has denied its forces were responsible for any of the bodies found in the mass graves and has called for the United Nations to investigate reports of violations committed by pro-GNA forces in Tarhouna.