North Korean authorities arrested Kim Dong Chul on suspicion of conspiring against the state.
Kim Dong Chul told CNN: "I committed an act of espionage against North Korea. I gathered information about its nuclear programme and military facilities.
"I bribed a local resident, an ex-soldier with military access. He handed over information, I hid it in my car, and secretly brought it to China.
"It's time for the US government to withdraw its hostile policy against North Korea.
"I'm asking the US or South Korean government to rescue me."
Mr Chul was forced to speak in Korean and through a state-provided translator. It is unclear whether his confession and other statements are genuine or made under duress.
He also said he was arrested three months ago with a USB drive, camera and documents with informaiton about North Korea's nuclear programme.
South Korea has denied accusations Kim Dong Chul was spying for the country.
Ma Young-ae, a North Korean defector and missionary who travelled with Mr Chul in 2007, has told Reuters he was a US pastor based in Virginia who made regular trips to North Korea to deliver medical aid for poor Northerners.
She said: "I was shocked to see his face on TV.
"He told the churches that he was a missionary working on North Korea and sending stuff from China into the North to help poor North Koreans."
The US state department has refused to comment on Kim Dong Chul's case, saying that "speaking publicly about specific purported cases of detained Americans can complicate our tireless efforts to secure their freedom."
Mr Chul still has no trial date.
He is the second western Christian to make headlines as a North Korean detainee in recent weeks.
Canadian Hyeon Soo Lim was sentenced to a life of hard labour after he publicly confessed to conspiring against the state. Observers doubt his confession was genuine.
Mr Lim appeared this week in an interview saying that he dug holes for eight hours a day, six days a week. He also reported that he was fed three times a day and received medical treatment when necessary.
