The UN's top human rights official has said that as many as 200,000 people are being held in North Korean political prison camps that are rife with torture, rape and slave labour, and that some of the abuses "may amount to crimes against humanity".
Navi Pillay called on Monday for an international investigation into alleged abuses, which she described as having "no parallel anywhere else in the world".
"Because of the enduring gravity of the situation, I believe an in-depth inquiry into one of the worst - but least understood and reported - human rights situations in the world is not only fully justified, but long overdue," Pillay said.
She said that there had been no improvement since Kim Jong-un took power a year ago following the death of his father, Kim Jong-il.
Living conditions in the camps are reported to be "atrocious" with insufficient food, little or no medical care and inadequate clothing for inmates, she said.
The UN rights chief said, "The death penalty seems to be often applied for minor offences and after wholly inadequate judicial processes, or sometimes without any judicial process at all."
In the comings weeks, Japan is weighing whether to submit a resolution on North Korea to the UN Human Rights Council, whose session begins on February 25, diplomats have said.
North Korea has ignored a series of council special investigators for years, denying them entry.
On Monday, Ri Jang Gon, deputy ambassador at North Korea's mission to the United Nations in Geneva, categorically dismissed the allegations.
"We totally reject the news release. Our country doesn't have such kind of crimes," Ri told Reuters news agency.
Pillay said she regretted that international concerns over North Korea's nuclear programme and rocket launches were overshadowing "the deplorable human rights situation" in the reclusive state.
Last week, former New Mexico state governor Bill Richardson and Google executive Eric Schmidt failed to secure the release of a Korean-American held in North Korea.
The timing of their trip was criticised by the US state department, coming after Pyongyang carried out a long-range rocket launch last month, something Washington considers a provocative test of ballistic missile technology.
Pillay said that she had met two survivors of North Korea's labour camps in Geneva in December and their personal testimonies had been "extremely harrowing".
One of them, a man, had been born in a camp and spent 23 years there, subjected to torture and forced labour.
At 14, he was made to watch his mother and brother being executed.
"One mother described to me how she had wrapped her baby in leaves when it was born and later made her a blanket by sewing together old socks," she said.