Amnesty International says satellite images indicate that North Korea's network of political prison camps has grown over the last decade.
The human rights group said a comparison of the latest satellite images from those in 2001 suggests there has been a "significant increase" in the scale of the camps, where inmates are reportedly held in "horrific" conditions.
The images show four of the six camps located in South Pyongan, South Hamkyung and North Hamkyung provinces, Amnesty International said.
"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," said Sam Zarifi, Amnesty International's Asia-Pacific director.
Zarifi said the country can "no longer deny the undeniable," and urged the country to close the camps immediately.
"These are places out of sight of the rest of the world, where almost the entire range of human rights protections that international law has tried to set up for last 60 years are ignored," Zarifi said.
Amnesty said there were several accounts of people eating rats to survive in the camps, which hold an estimated 200,000 people.
'If you only finished half of your assigned task, you would only be given half of your food.' —Jeong Kyoungil, camp detainee
Many of the inmates are believed to have been sent to the camps simply because a relative has been detained, while others don't know what they have been accused of, the rights group said.
Jeong Kyoungil told the organization that he was detained in a camp in Yodok from 2000 to 2003. He said the work day started at 4 a.m. and ended at 8 p.m., with short breaks for small meals.
"If you only finished half of your assigned task, you would only be given half of your food," he told Amnesty in April.
Amnesty said former detainees reported that prisoners at Yodok are forced to work in conditions "approaching slavery, and are frequently subjected to torture and other cruel, inhumane, and degrading treatment."