There was no direct word from 85-year-old Merrill Newman, and his alleged apology, which was dated November 9, couldn't be independently confirmed. Pyongyang has been accused of previously coercing statements from detainees.
North Korean authorities released video showing Newman reading the apology.
The statement, carried in the North's official Korean Central News Agency, said the war veteran allegedly attempted to meet with any surviving soldiers he had trained during the Korean War to fight North Korea, and that he admitted to killing civilians and brought an e-book criticising North Korea.
The apology can be seen as Pyongyang taking steps needed to release Newman, said Yoo Ho-Yeol, a professor of North Korea studies at Korea University in Seoul.
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North Korea likely issued the confession in the form of an apology to resolve Newman's case quickly without starting legal proceedings, Yoo said.
North Korea is extremely sensitive about any criticism and regularly accuses Washington and Seoul of seeking to overthrow its authoritarian system through various means, claims the US and South Korea dismiss. The State Department has repeatedly warned Americans about traveling to the country, citing the risk of arbitrary detention.
His travelling companion seated next to him, neighbour and former Stanford University professor Bob Hamrdla, was allowed to depart.
Newman's son, Jeffrey Newman, said his father wanted to return to the country where he spent three years during the Korean War.
North Korea has detained at least six Americans since 2009, including two journalists accused of trespassing and several Americans, some of whom are of Korean ancestry, accused of spreading Christianity.
North Korea sees missionary work as a Western threat to its authoritarian government.