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North Korea is pushing ahead with plans to expand its infamous labor camps for political prisoners, according to a report released Wednesday by Amnesty International.

The human rights group released satellite images reportedly showing continued expansion at two of the country's largest political prison camps, including new housing blocks, production facilities, and reinforced perimeter security.

“The gruesome reality of North Korea’s continued investment in this vast network of repression has been exposed. We urge the authorities to immediately and unconditionally release all those prisoners of conscience held in political prison camps and close the camps immediately,” said Rajiv Narayan, Amnesty International’s East Asia Researcher.

The satellite images, taken in May 2013, indicate a slight increase in population at Camp 16, the largest political prison camp in North Korea, with new housing blocks visible. In 2011, an estimated 20,000 people were believed be imprisoned at the camp, Amnesty said.

North Korea refuses to allow United Nations researchers and other outside groups access to its detention facilities and denies the existence of political prison camps.

The report also details testimony from a former security guard at Camp 16, identified only as Mr. Lee, who has never spoken publicly before about conditions in the facility.

Lee said detainees were forced to dig their own graves and were then killed with hammer strikes to their necks. He told Amnesty in an interview that he witnessed prison officers strangling detainees and then beating them to death with wooden sticks.

According to Lee, women were killed after being raped. "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 was quoted as saying.

Kim Young-soon, a former detainee in Camp 15 from 1980 and 1989, recalled a public execution she witnessed of two detainees who were caught trying to escape.

"They were brought to a stage after they were badly beaten. The prisoners were tied to wooden stakes and shot three times in their head, chest and feet,” she said.

According to Amnesty, the satellite images show significant industrial activity at Camps 16 and 15, including mining and logging. The group has not been able to verify the prison population at Camp 15, located in central North Korea about 75 miles from Pyongyang.

A report released by the Washington-based Committee for Human Rights in North Korea in September said thousands of prisoners may have died following the 2011 closure of Camp 22 in North Hamyong province.

The group, citing an account from a North Korean defector, said the notorious camp once held an estimated 30,000 inmates, but numbers rapidly deteriorated to 3,000 amid a food shortage.

Amnesty International said it has shared its latest findings with the U.N., which launched an investigation into human rights abuses in North Korea in July.