SEOUL (REUTERS) - A South Korean missionary held in North Korea said he had committed crimes against the state in a bid to establish an underground church, according to video footage provided by the North's state news agency on Thursday.

The South Korean missionary, identified by the North as Kim Jong Uk, 50, spoke at what appeared to be a stage-managed news conference from beneath portraits of North Korea's former leaders Kim Jong Il and Kim Il Sung.

His sudden appearance was a surprise after signs of warming ties between the two Koreas evidenced by the reunions of families split by the 1950-53 war between the two sides and Seoul's humanitarian offer to support vaccines against foot-and-mouth disease for the North.

"Since I first arrived in Dandong, China, in August 2007, I had thought a lot about sneaking into Pyongyang," Kim said in the video.

Kim, wearing a dark suit and blue tie, said he had first met North Koreans while working as a missionary in China.

"I told them we should build God's nation and break down the North's regime and political system. I gave them an anti-Republic education and orders to go back and build an underground church," he said.

North Korea said the missionary is a South Korean spy who was secretly trying to overthrow the state.

Pyongyang said in November it had arrested a South Korean spy and was investigating him on charges of espionage, a rare report of a secret agent being captured by either neighbour.

After the broadcast, the South Korean government urged Pyongyang to free the missionary.

"It is hard to understand that (North Korea) calls our national, who is doing purely religious activities, an anti-state criminal," Mr Kim Eui Do, a spokesman of the South's Unification Ministry that handles inter-Korea affairs, told a news briefing.

North Korea espouses freedom of religion but ranks as one of the world's most oppressive regimes in that regard. The United Nations this month cited a lack of religious freedom in a state whose human rights abuses it likened to those of Nazi Germany.

North Korea has held Korean-American missionary Kenneth Bae for more than a year and convicted him of trying to overthrow the state. A North Korean court sentenced Bae to 15 years' hard labour and efforts by Washington to secure his release have been thwarted.

Australian missionary John Short was detained this month by North Korean authorities in Pyongyang for carrying religious materials, his family said.