North Korea is believed to be operating a total of four political prison camps, where up to 65-thousand people are being forced to perform harsh physical labor, according to a recent report.

The Korea Institute for National Unification(KINU), a South Korean government-backed think tank, released on Friday an update to its 2013 report on North Korea’s political prisoner detention facilities, which includes U.S. commercial satellite imagery and testimonies from North Korean defectors.

According to the report, there are currently four political prison camps in operation–Camp 14, Camp 16, Camp 18 and Camp 25–three of which are being managed by North Korea’s national security agency, and one by the national police.

They are located in South Pyongan Province and North Hamgyong Province.

The report estimated that anywhere from 53-thousand up to 65-thousand people are being detained in the four camps, marking a significant decline compared to the estimate of up to 120-thousand prisoners earlier reported by the institute.

According to KINU, the decline in prisoner numbers does not suggest an improvement in North Korea’s human rights situation, but rather is the result of the closures of other camps and inmates dying amid poor living conditions.

North Koreans are subject to imprisonment and forced physical labor at these facilities when they act against regime leader Kim Jong-un's orders or the ruling party, commit crimes against the state or engage in what it deems as subversive acts, including watching South Korean drama and movies.

North Korea has long denied the existence of political prison camps, but indirectly recognized them during last November’s Universal Periodic Review at the United Nations office in Geneva.