Men, women and children are being kidnapped and held for months as slaves by militias in eastern Democratic Republic of the Congo (DRC).

Healthcare professionals working for Médicins sans Frontières in the gold and diamond mining regions of Okapi forest, Orientale province, say they have treated hundreds of women who had been seized from villages and held as sex slaves, many of whom have life-threatening injuries from sustained abuse. Men and children are also being kidnapped and made to work in the mines.

"They describe what they have lived through as hell," said Ana Maria Tijerino, an MSF psychologist who works in the nearby town of Nia Nia, to which thousands of people have fled to escape the violence. "I have trouble believing that such a level of horror is possible. Victims have been held as sex slaves – sometimes for months at a time – and sexually assaulted violently by several men, several times a day, often in front of their parents, husbands or relatives."

Between May and early July, MSF staff in Nia Nia provided 3,586 consultations to local people. They also gave psychological support to 143 women, three men and two children who had experienced sexual violence, and to more than 36 survivors of other types of violence, including torture and being forced to witness atrocities against relatives. Last month, the team treated 20 women in a single village who had been raped.

MSF says security and the rule of law have collapsed in recent months and that the military are struggling to overcome militiamen, many of whom are former poachers with detailed knowledge of Okapi's dense forested areas.

"After a militia leader was killed by the military in April, the level of violence, and the brutality, increased significantly, targeting both the mining communities and people in the surrounding villages," Kevin Coppock, MSF's head of mission in Orientale province, said. "Militia members simply show up, steal what they can and take men and women out of local communities who are then kept captive under the most horrendous conditions for months at a time."

Coppock said women who had escaped had sought help at MSF clinics for debilitating injuries and sexually transmitted diseases. "Rape continues to be endemic across the country, but if a woman can get medical attention within 72 hours of an attack taking place they have a much improved chance of recovery," he said.

"After weeks of being kept prisoner and suffering daily sustained attacks, there is a limit to what we can do for them. There is a lot of public attention on the problem of sexual violence in the DRC but the experiences of these women who have gone through sexual slavery is difficult to describe."

MSF says the threat of kidnap has put huge psychological strain on people in the area. "[Many] live in fear of having to return to the mines under the yoke of their attackers, which adds to their stress," Tijerino said. "They are afraid because they know they cannot survive economically without working in the mines. Violence and sexual violence are nothing new in DRC, but for the victims, these atrocities are not normal. No one should have to accept violence on this level."