More than 260 female inmates were sexually assaulted during an attempted mass escape from the Democratic Republic of Congo's Makala Central Prison this month, an internal U.N. report reviewed by Reuters showed.

At least 129 people were killed when prison guards used live fire against inmates trying to break free from the Kinshasa prison, which official figures say has a capacity of 1,500 prisoners, but housed more than 15,000 people.

The government had said after the attempted breakout in the early hours of Sept. 2. that women had been sexually assaulted, without providing a figure.

But an internal U.N. report seen by Reuters on the incident said 268 out of 348 women incarcerated at Makala had been subjected to sexual violence, including rape, while the chaos unfolded. Of these, 17 were younger than 19.

The report had detailed numbers of the victims as each needed post-attack kits and emergency contraception within 72 hours.

A spokesman for the government did not immediately respond to requests for comment on Thursday.

One victim who did not wish to be named for fear of reprisals told Reuters by phone that at least a dozen inmates had raped her.

"I saw the male prisoners throwing themselves at the female prisoners, it was horrible," said another female prisoner who witnessed the events, adding that even elderly women had been attacked.

She also asked to remain anonymous for safety concerns.

They said prisoners attacked the block most of the women were housed in, one of seven out the prison's 11 blocks that was vandalised in the breakout attempt.

"This is the worst case we have seen in terms of the number of rapes during an escape," said Congolese rights defender Emmanuel Cole, who has been monitoring prisons since 1997.

He said the U.N. figures were likely to be an underestimate, as some of the victims may have been too afraid of repercussions to speak.

President Felix Tshisekedi has ordered an investigation into the Makala prison break and a review of the main prisons to reduce overcrowding. No escapes were ultimately reported in the incident.