Satellite images of the camps, released by Amnesty International, showing four of the six camps located in vast wilderness sites in South Pyongan, South Hamkyung and North Hamkyung provinces.
A comparison with satellite pictures from 2001 indicated a significant increase in the scale of the camps, which are believed to have been operating since the 1950s, it said.
The human rights group spoke to a number of people, including former inmates from the political prison camp at Yodok, as well as guards in other camps, revealing what it said were horrific conditions.
Thousands of people are believed to be held as 'guilty-by-association' or sent to the camps simply because one of their relatives has been detained. A significant proportion don’t even know what crimes they’re accused of.
The former detainees said prisoners were forced to work in conditions close to slavery and were frequently subjected to torture and other cruel, inhumane, and degrading treatment.
All the detainees at Yodok had witnessed public executions, it said.
Furthermore, food is scarce in the camps. Amnesty International said it had been told of several accounts of people eating rats or picking corn kernels out of animal waste to survive.
Amnesty called on North Korea, one of the world's most secretive states, to close all political prison camps and to release all prisoners of conscience.
'North Korea can no longer deny the undeniable. For decades the authorities have refused to admit to the existence of mass political prison camps,' said Sam Zarifi, Amnesty International's Asia Pacific director.
'As North Korea seems to be moving towards a new leader in Kim Jong-un and a period of political instability, the big worry is that the prison camps appear to be growing in size,' he said, referring to the son and presumed successor of Kim Jong-il. Read More