SEOUL, South Korea - North Korea said today that it had detained an American man who illegally entered the country last week through the border with China.

It did not identify the man, who it said was under investigation and had crossed the border Thursday. But the report comes as South Korean activists say American missionary Robert Park, 28, slipped across the frozen Tumen River into North Korea from China last week in an effort to call attention to the reclusive country's human-rights conditions.

The activists had said that Park entered North Korea on Friday.

Park was carrying a Bible and written appeals calling for an end to repression and leader Kim Jong Il's rule, said Jo Sung Rae, an activist in Seoul with Pax Koreana, an organization that promotes human rights in North Korea.

Jo quoted two Korean defectors who were helping Park as saying they heard people talking loudly across the river soon after he went over and so assumed that he was quickly detained.

Those who know Park say he is "unusually serious" about his Christian faith, with an intense devotion to prayer and ending suffering in North Korea.

Seoul-based activists say that Park had become a fixture over the last year in local circles advocating North Korean human rights and that he stood out for his religious fervor and passion for the cause.

Park's father, Pyong Park, described him as fearless.

"He was not afraid to die," said the elder Park, who lives in Encinitas, Calif. "What he wanted was the whole world to know of North Korea's situation."

Human-rights conditions in the country, where Kim rules with an iron fist and allows no dissent, are considered among the worst in the world. North Korea holds 154,000 political prisoners in six large camps, according to South Korean estimates. Pyongyang denies the existence of any repression.

South Korean analysts have been divided on what fate may await Park. Some have said North Korea would react harshly to such a direct assault on its political system, while others said it was likely to see him as a nuisance and may deport him.

Lee Sang Hyun of the Sejong Institute, a private think tank, said that with North Korea and the United States now engaged in dialogue, Pyongyang was unlikely to hold Park for long.

Pyong Park and his wife, Helen, Korean immigrants who met in the United States, said Sunday that they last heard from their son in a Dec. 23 e-mail.

"Know that I am the happiest in all my life, incredible miracles are happening for the liberation of North Koreans right now," Park wrote. "I am thankful to Jesus because of the opportunity to serve His holy purpose."

The U.S. Embassy in Beijing had said earlier that the U.S. Consulate in Shenyang in northeastern China was talking with Chinese authorities to try to confirm Park's whereabouts.