Helen Park, right, mother of missing missionary, Robert Park, is comforted by a friend at the Palomar Korean Church in San<\p>Marco, Calif., where friends gathered Sunday in support of the parents whose missionary son is missing in North Korea.

SEOUL, South Korea — North Korean border guards apparently detained an American missionary as soon as he walked into the communist nation in an effort to call attention to Pyongyang's human rights abuses, an activist said today.

Robert Park, 28, slipped across the frozen Tumen River into the North from China on Christmas Day carrying letters calling on North Korean leader Kim Jong Il to shut down the country's political prison camps and step down from power. There has been no word from him since.

Jo Sung-rae of the Seoul-based activist group Pax Koreana cited a person who witnessed Park's crossing as saying he heard people speaking on the North Korean side as soon as Park crossed over alone.

Jo quoted the person, one of two people who guided Park, as saying visibility was poor. "But he said he heard people talking loudly when Robert arrived there," Jo added. "I think they were border guards and Robert was taken into custody immediately."

North Korea's state-run media has not mentioned Park or any illegal entries into the country by foreigners. The U.S. State Department and the U.S. Embassy in Beijing said they were aware of the incident but had no details.

U.S. Embassy spokeswoman Susan Stevenson in Beijing said today the U.S. Consulate in Shenyang in northeastern China is treating Park's situation as a "welfare and whereabouts" case and is talking with Chinese authorities. She did not elaborate.

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

Park's letter to Kim Jong Il, one of two he was carrying, asked that he open the North's borders to humanitarian assistance and put an end to political prison camps.

"Please close down all concentration camps and release all political prisoners today," said the letter, according to a copy posted on Pax Koreana's Web site.

North Korea holds some 154,000 political prisoners in six large camps across the country, according to South Korean government estimates.