UN experts are accusing a warlord in the Democratic Republic of Congo of leading horrific gang rapes in South Kivu during which women were held captive in a cave and treated like sex slaves, according to a confidential report obtained by AFP on Friday.The group of experts on the DRC said in the report to the Security Council that the overall situation was "volatile" as the country held an elections to be on Sunday."While the upcoming elections have continued to raise security concerns, the group has not found evidence of direct involvement of armed actors in the electoral process," said the 61-page report sent to the council earlier this month. Based on interviews with victims, a witness, local authorities and other sources, the experts found that the Raia Mutomboki Kokodikoko faction had gang-raped 17 women in September and subjected them to torture and sexual slavery."The group concluded that Masudi Alimasi Kokodikoko, leader of a Raia Mutomboki faction in Shabunda territory, was a lead perpetrator of the gang rapes of at least 17 women in September 2018," said the report.The armed group abducted the women, aged between 15 and 70, during raids on villages in the Lubila region of South Kivu on September 8-9, the report said.The women and girls were held captive in "a big stone cave" in the forest and repeatedly raped during four days by the Raia Mutomboki fighters, it said."They also introduced various tools into the vaginas of several of the women, severely injuring some. Women who had resisted being assaulted were additionally mistreated," said the report. Dancing and singing "Raia Mutomboki elements shouting 'Tchai!Tchai!' (Tea!Tea!) usually announced the beginning of the rapes." Rebels would then dance and sing songs "described as full of insults" and praising their chief."Kokodikoko chose the women he preferred, usually selecting the youngest ones, and raped them first. He raped at least nine women. It was only after he had raped the selected women that he authorised or ordered his elements to rape the women," the report said.The mass rapes allegedly involved child soldiers, according to victims who said boys aged 15 and 16 were in the cave during the violence. South Kivu has been wracked by sexual violence in the decades of conflict involving armed groups in the eastern DRC.Congolese surgeon Denis Mukwege was awarded the Nobel Peace Prize this year for helping women recover from the trauma of rape. During a phone call with the UN experts, Kokodikoko denied any wrongdoing and rejected allegations of mass rapes by his faction, the report said. The experts recalled that perpetrators of sexual violence could face sanctions under UN resolutions and urged authorities to investigate and prosecute those responsible.Earlier on Friday, the DRC army said it had arrested a militia chief accused of systematic rapes in South Kivu. Isaac Chirambiza, who was detained on Tuesday, heads another faction of Raia Mutomboki, which portrays itself as defenders of the Congolese against the FDLR, a Rwandan Hutu group that has been fighting in eastern DRC for decades.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.