SEOUL, South Korea - North Korea said yesterday that it has detained an American man who illegally entered the country last week, following reports that a 28-year-old missionary from Arizona went to the communist nation on a mission to improve its human-rights record.

The North's official Korean Central News Agency said in a brief dispatch that the American was detained and under investigation after illegally entering through the North Korea-China border last Thursday. It didn't identify the American man.

However, the report comes as South Korean activists say that American missionary Robert Park, 28, slipped across the frozen Tumen River into North Korea from China last week with letters calling for a change in North Korea's leadership and an end to political prison camps.

The Rev. John Benson, the pastor of Life in Christ Community Church in Park's hometown of Tucson, Arizona, said that he was happy to hear that Park was alive.

"To hear it confirmed is great," Benson said. "I pray he comes back in one piece. That would be the best-case scenario.

"He did this to bring awareness to the situation in North Korea, alert the churches around the world to the atrocities and human rights violations, not pay lip service and do something about it," Benson said. "Drastic situations call for drastic measures."

Jo Sung-rae, of the Seoul-based activist group Pax Koreana, said yesterday that North Korean border guards apparently detained the missionary as soon as he walked into the communist nation.

Park was carrying letters calling on North Korean leader Kim Jong Il to shut down the country's political prison camps and step down from power, Jo said.

The activist quoted one of two people who guided Park as saying that he heard people who apparently were border guards speaking loudly as soon as Park crossed the border and that he believed that the missionary was taken into custody immediately.

Park's crossing comes just months after North Korea freed two U.S. journalists who had been arrested in March and sentenced to 12 years of hard labor for trespassing and "hostile acts."

North Korea holds some 154,000 political prisoners in six large camps across the country, according to South Korean government estimates. Pyongyang has long been regarded as having one of the world's worst human-rights records, but it denies the existence of prison camps.