More than 100 female prisoners were raped and then burned alive during a jailbreak in the Congolese city of Goma, according to the UN.
Hundreds of prisoners broke out of Munzenze prison last Monday, after fighters from the M23 rebel group began to take over the city.
Between 165 and 167 women were assaulted by male inmates during the jailbreak, an internal UN document seen by the BBC says.
The report states that most of the women were killed after the inmates set fire to the prison.
Goma, a major city of more than a million people, was captured after the Rwanda-backed M23 executed a rapid advance through eastern DR Congo.
The city was plunged into chaos, with bodies lying in the streets and missiles reportedly flying over residential homes.
Footage from last week’s jailbreak showed people fleeing from the building as smoke rose in the background. Heavy gunfire could also be heard.
More than 2,000 people were killed as the M23 clashed with the Congolese forces and their allies, DR Congo’s government has said.
The UN says at least 900 people were killed and almost 3,000 injured. It was unclear why the UN and DR Congo’s death tolls vary.
Earlier this week, the rebels announced a ceasefire on humanitarian grounds.
However, the M23 launched a new offensive on Wednesday, sources say, reportedly capturing the mining town of Nyabibwe.
Nyabibwe is about 100 km (60 miles) from Bukavu – the east’s second-largest city, and the reported target of the rebels’ most recent advance.