South Korea's presidential office confirmed Thursday that six of its citizens have been captive in North Korea for years, after President Lee Jae Myung appeared unaware of their plight during a briefing with foreign media.

Asked Wednesday about South Koreans detained in the North, Lee replied: "It's my first time ever hearing about this."

Lee's office later followed up with a statement saying that six nationals -- including Christian missionaries and North Korean defectors -- have been held since their arrests "between 2013 and 2016 on charges of espionage, among others".

Four of them have been named by Pyongyang, which has accused them of espionage -- a charge carrying severe penalties, including death, in the authoritarian country.

"In the current situation, where inter-Korean dialog

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