South Korea demanded Friday that North Korea release its nationals being held in the communist country, saying the arrests were "unilateral" and used for propaganda.

Earlier in Pyongyang, a South Korean man named Ko Hyon-chol held what appeared to be a stage-managed press conference in front of foreign journalists and diplomats to "confess" to trying to kidnap North Korean orphans, according to the AFP.

Ko, 53, defected to the South in 2014 after fleeing the North a year earlier, the report said.

"We strongly demand that North Korea swiftly release our detained nationals, including Mr. Ko Hyon-chol, and return them to us without delay," the South's unification ministry said in a statement.

Three other South Koreans -- Kim Jeong-wook, Kim Kuk-gi and Choi Chun-gil -- have also been held in the North since their arrests in 2013 and 2014.

"We express deep regret at the North's acts of unilaterally arresting our nationals and using them in press conferences and other forms of propaganda warfare," the ministry said.

Ko told the press conference he was ordered by South Korea's spy agency to abduct North Korean orphans in exchange for US$10,000 each.

He was arrested in the North in May after crossing the border from China.

Pyongyang accuses the South's National Intelligence Service of kidnapping 12 North Korean women from a restaurant in China in April. Seoul insists they defected to the South of their own will. (Yonhap)