The two Koreas technically remain at war. North Korea this year stepped back from the armistice agreement ending the 1950s conflict amid growing tensions with its adversaries in Seoul.
The South Korean Ministry of Unification estimated at least 30 of the 516 South Koreans detained in North Korea were police officers and soldiers, the Yonhap News Agency in Seoul reported Monday.
North Korea's human rights record has been overshadowed by concerns over its nuclear weapons program. A report from John Hopkins University last week said North Korea has a policy of "low-level lethal testing of chemical agents on unwilling human subjects," including political prisoners.
Human Rights Watch said as many as 200,000 people are in the North Korean gulag system. The rights organization said North Korea has hidden a system of systematic abuses behind "bluster and defiance."
South Korean President Park Geun-hye said last week she was concerned Pyongyang was putting its people in jeopardy by focusing too much on nuclear weapons.