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 a letter calling on North Korean leader Kim Jong Il to shut down the country’s political prison camps. 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 crossing into the North as saying that voices were heard on the North Korean side as soon as he crossed over. Jo quoted the person as saying, “I think they were border guards, and Robert was taken into custody immediately.”