Kim Jong-uk, 50, said he was arrested after entering via China with religious materials in October.
Religious activity is restricted in North Korea, with missionaries arrested on multiple occasions in the past.
Foreign nationals arrested in North Korea sometimes make public confessions which they later say were under duress.
Merrill Newman, an 85-year-old US national, was briefly held last year. He was freed after confessing to committing crimes during the Korean War – a statement he said was given under duress.
In his first public appearance since his arrest, Kim Jong-uk said he wanted to let his family know he was in good health.
He said he acted “under directions” from South Korea’s National Intelligence Services (NIS), setting up an underground church in Dandong, China, to collect information on life in North Korea to send back.
“I was thinking of turning North Korea into a religious country, and destroying its present government and political system,” Kim Jong-uk also told the news conference.
One report said Kim Jong-uk had been working in Dandong for seven years helping North Korean refugees.
Kim Jong-uk said he was unsure of his punishment and asked that he be released.
The North Korean state media in November said it had arrested an unnamed South Korean “spy”, a charge which South Korea’s intelligence agency denied.
On Thursday, South Korea’s Unification Ministry urged North Korea to release and repatriate Kim Jong-uk.
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