A satellite image showing the area of political prison camps in central North Korea. Photograph: Digitalglobe/Amnesty International/AFP/Getty Images

North Korea's political prison camps have expanded substantially over the last decade and hold 200,000 people, according to Amnesty International.

The human rights group says satellite imagery shows that locations identified as prison camps by defectors have grown rapidly in recent years. Former detainees and guards who subsequently escaped from the country described horrific conditions including torture and widespread malnutrition, Amnesty said, with every former inmate saying they had witnessed executions.

Thousands of prisoners are thought to be held purely for guilt by association under a system of collective punishment that holds relatives responsible when an individual breaks the law.

"Hundreds of thousands of people exist with virtually no rights, treated essentially as slaves, in some of the worst circumstances we've documented in the last 50 years," said Sam Zarifi, Amnesty'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."

Satellite images showed four of the six camps sprawling over large tracts of land in remote rural areas of South Pyongan, South Hamkyung and North Hamkyung provinces.

A comparison with images taken 10 years ago showed that they had grown significantly, Amnesty said, with 15 more guardhouses at Yodok alone. It combined the evidence of additional buildings with former detainees' descriptions of conditions at the camps to arrive at its estimate of political prisoner numbers.

The South Korean human rights commission has also estimated a population of 200,000 while South Korean government sources have suggested around 150,000 people are held in the camps. North Korea has a population of around 24 million.

According to defectors, the camps consist of two areas. No one is ever released from the total control zones, for those alleged to have committed serious crimes such as those against the regime.

Those deemed to have committed less serious offences, such as being critical of government policy or illegally crossing the border, face sentences of between a few months and a decade in revolutionary zones.

Amnesty says only three people are known to have escaped total control zones and managed to leave North Korea. Most of its interviewees had been held in the revolutionary zone at a camp in Yodok.

A devastating famine in the mid-1990s killed hundreds of thousands in North Korea and the country has suffered food shortages ever since, but conditions appear to have been particularly gruelling in the camps.

Former inmate Jeong Kyoungil, who was detained in Yodok from 2000 to 2003, told Amnesty that prisoners received three meals of 200g of corn gruel a day, with food withheld if they failed to finish their work.

"Seeing people die happened frequently – every day. Frankly, unlike in a normal society, we would like it rather than feel sad because if you bring a dead body and bury it, you would be given another bowl of food. I used to take charge of burying dead people's bodies," he said.

Others reported torture, with detainees forced to spend a week or more in a cube-shaped cell too small in which to stand or lie down. North Korea, which does not acknowledge it has political prison camps, has not responded to the report.