Amnesty International says new satellite images and testimony from a former inmate show North Korea is holding about 200,000 people in its huge network of political prison camps where torture is rampant and conditions near slavery.

The human rights group based in London released on Wednesday recent satellite photos which show four of six camps located in the impoverished country's South Pyongan, South Hamkyung and North Hamkyung provinces.

North Korea has long been regarded as having one of the world's worst human rights records, but it rejects outside criticism and denies the existence of prison camps.

"North Korea can no longer deny the undeniable," Sam Zarifi, Amnesty International Asia Pacific director, said in a statement.