Kim Tong Chol told a press conference in Pyongyang that he had collaborated with and spied for South Korean intelligence authorities in a plot to bring down the North's leadership and tried to spread religious ideas among North Koreans.
Describing his acts as "shameful and ineffaceable," Kim said he feels sorry for his crime and appealed to North Korean authorities to show him mercy by forgiving him.
He was born in South Korea and became a naturalized US citizen. In an interview with CNN in January, Kim said he lived in Fairfax, Virginia, before moving in 2011 to Yangji, a city near the Chinese-North Korean border.
North Korean authorities often arrange press conferences for US and other foreign detainees in which they read statements to acknowledge their wrongdoing and praise the North's political system. Those detainees have said after their releases that they were coached or coerced on what to say.
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South Korea's National Intelligence Service, the country's main spy agency, said Kim's case wasn't related to the organization in any way and offered no further comment.
He tearfully confessed at his press conference to the attempted theft, which would be grounds in North Korea for a subversion charge.
The US government condemned the sentence and accused North Korea of using such American detainees as political pawns.
The North's actions on the two American detainees came as it faces mounting pressure from the United States, South Korea and their allies following its nuclear weapons test and long-range rocket launch earlier this year.