North Korea's largest prison labour camps appear to be expanding as a report details the amount of alleged rape, torture and killings which occur inside.
Satellite images released by Amnesty International appear to show the camps, known as Kwanliso, augmented with new housing blocks and production facilities.
The human rights charity also spoke to a former security guard at one of the political prisons, known as Camp 16.
The guard, known only as Mr Lee, told Amnesty International that woman are raped and killed in the camps, adding that people are starved and forced to dig their own graves and are treated "worse than animals".
The report now fears the camps are expanding and are calls for North Korea to close them down. The secretive state has always denied the existence of the camps, which are estimated to hold as many as 200,000 people.
The report's author Rajiv Narayan said: "Under its new leader Kim Jong-un, North Korea is violating every conceivable human right.
"The camps are a gruesome and powerful tool at the heart of a vast network of repression.
"People are sent to the political prison camps without charge, let alone a trial, many of them simply for knowing someone who has fallen out of favour.
"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."
The images released by Amnesty International, taken over a two-year period, show the areas believed to house Camp 15 in the south of the country and Camp 16 in the north.
Amnesty estimates the size of Camp 16 to be 216 square miles - three times the size of Washington, DC - with 20,000 men, woman and children held inside. The group says the photos show how the camp is expanding, with additional economic activity such as mining, logging and agriculture.
'Worse off than animals'
Lee, who has never spoken publicly before, discussed the alleged brutality and execution methods which occurred in the camp while he worked there in the 1980s until the mid-1990s.
He said detainees were forced to dig their own graves and were then killed with hammer blows to their necks. He also witnessed prison officers strangling detainees and then beating them to death with wooden sticks.
According to Lee, women also "disappeared" after being raped by prison guards. He told Amnesty: "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."
Former prisoners in Camp 15 also spoke of how they were subjected to forced labour - sometimes for as long as 12 hours - while severely malnourished.
"The purpose of prison camps is to oppress, degrade, and violate the inmates for as long as they are alive," Lee told Amnesty.
"The prisoners are only humans insofar as they can speak. In reality though, they are worse off than animals." he added.
Previous reports have also suggested inmates inside North Korea's camps are fed poison for experimentation, women have been forced to kill their own children and forced to stone each other to death and eat rats and frogs in order to survive.
Narayan added: "We are calling on the North Korean authorities to acknowledge the existence of the camps, close them, and grant unhindered access to independent human rights monitors like Amnesty International."