Amnesty International has urged North Korea to close two political prisoner camps, where it says torture is rampant and execution commonplace.
It quotes one former official as saying that inmates are forced to dig their own graves and women disappear after "servicing" officials.
Amnesty alleges that hundreds of thousands of people are held in detention facilities in North Korea.
Amnesty says it has passed its latest evidence to the UN Commission of Inquiry investigating human rights abuses in North Korea.
The rights group says it interviewed one former security official from Kwanliso 16 last month.
The official, referred to as Mr Lee, said prisoners were forced to dig their own graves and were then killed with blows to the neck.
Mr Lee said he witnessed prison officers strangling detainees and beating them to death with wooden sticks.
He added: "After a night of 'servicing' officials, women had to die because the secret could not get out. This happens at most of the political prison camps."
Kwanliso 15 covers 367 sq km (142 sq miles) and is in central North Korea, about 72km (45 miles) from the capital Pyongyang.
Kwanliso 16, near Hwaseong in North Hamgyong province, covers approximately 556 sq km.
Amnesty said it was not able to verify prisoner populations, but said there might have been a slight increase at Kwanliso 16 and a slight decrease at Kwanliso 15.
The report's author, Amnesty North Korea researcher Rajiv Narayan, said: "Under its new leader Kim Jong-un, North Korea is violating every conceivable human right.
"People are sent to the political prison camps without charge, let alone a trial, many of them simply for knowing someone who has fallen out of favour."
Mr Narayan added: "We are calling on the North Korean authorities to acknowledge the existence of the camps, close them, and grant unhindered access to independent human rights monitors like Amnesty International."