The United Nations said acts of genocide may have been committed in the DR Congo as it published a hotly-contested report Friday detailing massacres by foreign armies and rebels in the war-torn nation.

Rwanda, whose troops were at the centre of the most serious accusations, said it categorically rejected the report after it failed to have it suppressed while Burundi said it was designed to destabilise the region.

The Democratic Republic of Congo's government said it was "appalled" by the details in the report covering 1993 to 2003 and demanded justice for the victims.

"While it neither aims to establish individual responsibility, nor lay blame, the report -- in full candour -- reproduces the often shocking accounts by victims and witnesses of the tragedies they experienced," Navi Pillay, the UN high commissioner for human rights, said in the preface to the report.

"The report is intended as a first step towards the sometimes painful nonetheless essential process of truth-telling after violent conflict," she added.

Uganda and Angola also denied allegations against their armies.

The report, reworded in parts after a leak, said the "apparent systematic and widespread attacks... reveal a number of inculpatory elements that, if proven before a competent court, could be characterised as crimes of genocide", pointing in particular to attacks by Rwandan troops during 1996-1997.

"It was not a question of people killed unintentionally in the course of combat, but people targeted primarily by AFDL (Alliance of Democratic Forces for the Liberation of Congo)/APR (Rwandan Army)/FAB (Burundi army) and executed in their hundreds," it added.

The AFDL was led by Laurent Kabila, who proclaimed himself president after dictator Mobutu Sese Seko was overthrown. His son Joseph, now president of DR Congo, was fighting alongside.

The Rwandan government reacted furiously, saying it "categorically rejected" the report.

"The desire to validate the double genocide theory is consistently present throughout the report by mirror imaging the actors, ideology, and methods employed during the 1994 Rwandan genocide," the statement from Rwanda said.

The accusations of genocide are particularly contested by Kigali as its government has based much of its legitimacy on being the force that stopped the genocide in Rwanda in 1994.

Rwandan President Paul Kagame, who had earlier dismissed the report's claims as "absurd", was at the vanguard of the Rwandan force which drove the Hutu militias behind the 1994 genocide in his homeland across the border into eastern DR Congo.

Amnesty International's Erwin Van Der Borght said it is "quite appalling that states have attacked the work of the UN team," stressing that the report was a "very credible piece of work" with "very solid methodology."

"It's a step backward if neighbouring countries reject the report offhand because obviously they have a role to play -- many of their troops were directly involved in the conflict in the DRC," he said.

The report's language was already less assertive than in a leaked draft compiled by a team of investigators since 2008.

Words like "allegedly" or "apparently" have been inserted into the final version of descriptions of violations, as well as references to the involvement of the foreign armies during the 1996-1998 first Congo war.

However, Navi Pillay, the UN High Commissioner for Human Rights, told reporters in Geneva that "the substance [of the report] remains the same".

The UN report is the product of a "mapping exercise" that took more than two years to research and produce.

The report, compiled after interviews with over 1,280 witnesses, also included more paragraphs explaining the difficulties of proving genocide in court.

It said therefore that a full judicial inquiry was necessary to determine if the incidents amount to the crime of genocide.

In a statement at the UN headquarters in New York, the Congolese government's ambassador called for justice.

"The victims deserve justice and they deserve that their voices are heard by my government and by the international community," Ileka Atoki said, proposing possible mixed international-DR Congo courts to try the perpetrators.

Human Rights Watch's Carina Tertsakian urged for a "serious response from the UN and its member states."

"The UN Security Council should formally discuss the report and should insist that all the countries that are implicated in the report should help with efforts to bring justice," she told AFP.

Read the full UN report

Watch an interview with the UN High Commissioner for Human Rights Navi Pillay on the release of a 550-page report listing 617 of the most serious violations of human rights and international humanitarian law over a 10-year period, by both state and non-state actors in the Democratic Republic of the Congo (DRC).