North Korean security chiefs and possibly even Supreme Leader Kim Jong-un himself should face international justice for ordering systematic torture, starvation and killings comparable to Nazi-era atrocities, UN investigators said on Monday.

The investigators told Kim in a letter they were advising the UN to refer North Korea to the International Criminal Court (ICC), to make sure any culprits "including possibly yourself" were held accountable.

North Korea "categorically and totally" rejected the accusations set out in a 372-page report, saying they were based on material faked by hostile forces backed by the United States, the European Union and Japan.

The unprecedented public rebuke and warning to a ruling head of state by a UN Commission of Inquiry is likely to further antagonise Kim and complicate efforts to persuade him to rein in his isolated country's nuclear weapons programme and belligerent confrontations with South Korea and the West.

The UN investigators said they had also told Kim's main ally China that it might be "aiding and abetting crimes against humanity" by sending migrants and defectors back to North Korea to face torture and execution - a charge that Chinese officials had dismissed.

As referral to the Hague-based international court was seen as unlikely, given China's probable veto of any such move in the UN Security Council, thoughts were also turning to setting up some form of special tribunal on North Korea, diplomatic and UN sources told Reuters.

"We've collected all the testimony and can't just stop and wait 10 years. The idea is to sustain work," said one.

Michael Kirby, chairman of the independent Commission of Inquiry, told Reuters the crimes the team had catalogued were reminiscent of those committed by Nazis during World War Two.

"Some of them are strikingly similar," he said.

"Testimony was given ... in relation to the political prison camps of large numbers of people who were malnourished, who were effectively starved to death and then had to be disposed of in pots, burned and then buried ... It was the duty of other prisoners in the camps to dispose of them," he said.

Asked why the report had specifically mentioned Kim, Kirby told journalists: "A great deal of responsibility must lie on such a person. If you are at the centre, then you have power to change things."

"I do expect that the report will galvanise action on the part of the international community," he said.

Kirby, asked how many North Korean officials might be guilty of having committed the gravest crimes, replied: "The potential would be running into the hundreds, I would think."

The independent investigators' report, the size of a telephone directory, listed atrocities including murder, torture, rape, abductions, enslavement, starvation and executions.

"The gravity, scale and nature of these violations reveal a state that does not have any parallel in the contemporary world," it said. - Reuters