GENEVA (Reuters) - Mutilation, gang rape and killing documented in Congo’s Kasai region could be a harbinger of genocide, the U.N. torture investigator told Reuters on Wednesday, calling for action to prevent another Rwanda or Srebrenica.

Nils Melzer, U.N. special rapporteur on torture worldwide, said he was alarmed by a report issued by U.N. human rights experts on Tuesday which said Congolese rebels and government troops have committed atrocities including mass rape, cannibalism and the dismemberment of civilians.

The U.N. report - building on an earlier report accusing all sides of war crimes and crimes against humanity - catalogued gruesome attacks committed in the conflict in the central region of Kasai, which began in late 2016, involving the Kamuina Nsapu and Bana Mura militias and Congo’s armed forces.

“My greatest concern, however, is that what we are witnessing today may be only the prelude of what is still to come. In my view, Kasai already today bears the signature of Rwanda and Bosnia in the early 1990s,” Melzer told Reuters.

Congo’s army and allied Bana Mura militia opposed the Kamuina Nsapu militia in a partially ethnic conflict that erupted in 2016 and still simmers. It was one of a series of flare-ups to hit Congo around the time President Joseph Kabila refused to leave power when his mandate expired at the end of 2016.

Rwanda’s 1994 genocide saw 800,000 Tutsis and moderate Hutus killed in 100 days by the Hutu-led government and ethnic militias. The United Nations and major powers failed to halt the slaughter, despite cables from the field that it was looming.

Some 8,000 Muslim men and boys were massacred by Bosnian Serb troops under the command of former General Ratko Mladic at Srebrenica, a U.N.-designated “safe haven”, in July 1995, the worst mass killing on European soil since World War Two.

“Today Kasai is a hell that is just about to break loose,” said Melzer, a Swiss international lawyer serving in the independent U.N. post.

“Our experts have delivered the evidence and it is now up to the world leaders to get their act together and prevent the next genocide, the next exodus of millions to all corners of the world and the next unforgivable tragedy in human history.”

Major powers on the U.N. Security Council failed to heed warnings to halt ethnic violence in Rwanda or Bosnia but years later set up international tribunals to prosecute perpetrators.

At least 5,000 people have been killed in the Kasai region over the past two years and more than 1.4 million displaced, while “only a few low-level criminal suspects have been prosecuted”, Human Rights Watch said on Tuesday.

“The risks of further violence, abuse, and repression in the coming months are very real, with potentially devastating consequences throughout the region,” Laila Matar of the New York-based group told the U.N. Human Rights Council.