A new report has documented what life is like in a network of North Korean labour camps.
It presents a detailed picture of torture, forced abortions and public executions in a vast network of secret political camps.
The BBC says the research backs previous evidence that more than 150,000 political prisoners are being held in North Korea.
The Committee for Human Rights in North Korea, which is based in Washington DC, is calling for an international commission of inquiry.
Its researchers relied for their information on some 60 former prisoners and camp guards, who have escaped to South Korea.
The BBC reports they describe a complex web of prisons and work camps built to punish those seen as resistant to North Korea's system of total state control, including those who try to escape to South Korea.
The authors say that many inmates die from malnutrition and harsh working conditions as they labour in mines, factories and on farms.
Former inmates say they were sent to the camps - sometimes for decades - without any judicial process or often without hearing the charges against them.
Children and other family members of the accused can also be incarcerated in an effort to eradicate all criticism of North Korea's political system.
Witness accounts of public executions - mainly of prisoners who try to escape or are caught stealing extra food rations - are also documented.