Seoul AFP-JIJI

Satellite images of one of North Korea's largest political prison camps suggest that its inmate population is expanding, Amnesty International said Thursday in a report detailing rape and torture in the North's notorious gulag system.

The report by the London-based rights watchdog includes rare testimony from a former camp guard, as well as from former inmates about the brutality prevalent in the prison system.

"For Amnesty International, which has been investigating human rights violations for the last 50 years, we find North Korea to be in a category of its own," said Amnesty's East Asia researcher Rajiv Narayan.

North Korea denies the existence of the political prison camps, which according to independent estimates form a network holding somewhere between 100,000 and 200,000 people.

The images analyzed in the Amnesty report, taken from 2011 to 2013, are of Camp 15 in the south of the country and Camp 16 in the north.

Amnesty estimated the size of Camp 16 is 560 sq. km with around 20,000 prisoners.

A former security guard based at the camp from the 1980s until the mid-1990s, named only as Lee in the report, told Amnesty of the methods used to execute prisoners.

He revealed that detainees were forced to dig their own graves and were then killed with hammer blows to their necks.

The former guard said he also witnessed prison officers choking detainees and then beating them to death with wooden sticks.

Prison officials frequently raped women inmates who were then killed, he said.