A glimpse into Kim's expanding gulags

North Korea's largest gulag appears to be expanding its population, according to satellite images released by Amnesty International.

An Amnesty report included rare testimony from a former camp guard and 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 East Asia researcher Rajiv Narayan.

The former guard said detainees were forced to dig their own graves and were then killed with hammer blows to their necks.

Pyongyang denies the existence of the political prison camps, which, according to independent estimates, form a network holding between 100000 and 200000 people.

The images analysed were taken over a two-year period from 2011, and were of Camp 15 in the south of the country and Camp 16 in the north.

Amnesty estimated the size of Camp 16 at three times the size of Washington DC, with about 20000 prisoners.

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

He said he had witnessed prison officers strangling detainees and beating them to death with wooden sticks.

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

"After a night of 'servicing' the officials, the women had to die because the secret could not get out. This happens at most of the political prison camps," he told Amnesty