North Korean border guards apparently detained an American missionary when he walked into the communist nation in an effort to call attention to Pyongyang's human rights abuses, an activist said.
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 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 Park crossed over.
Jo quoted the person, one of two people who guided Park to the crossing, 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."
Members of Park's church in Tucson, Arizona, held services on Saturday and Sunday night to pray for his safe return, said the Reverend John Benson, the pastor at Life in Christ Community Church. About 70 people attended Saturday's vigil, he said.
Park's father, Pyong Park, quoted his son as saying before the journey he was "not afraid to die, as long as the whole world, all every nation pay attention to the North Korea situation, my death is nothing". The senior Park spoke to San Diego's KFMB television.
Jo said two guides, whom he described as North Korean defectors, filmed Park's crossing. But one of them is demanding payment for the footage and is refusing to hand it over.
Jo, who has been the source of most information about Park, initially requested anonymity, citing the sensitivity of the situation, but has now agreed to be named.
North Korea's state-run media has been silent on Park's case. The US State Department and the US Embassy in Beijing said they were aware of the incident but had no details.
Park, who is of Korean descent, was carrying a letter urging leader Kim Jong Il to step down and free all political prisoners, the activist said.
"I am an American citizen. I brought God's love. God loves you and God bless you," Park said in Korean as he crossed over near the northeastern city of Hoeryong, according to Jo.
Park was holding a Bible in his right hand and a piece of paper printed with a hymn text in the other, Jo said.
Park's crossing comes just months after North Korea freed two US journalists who had been arrested in March and sentenced to 12 years of hard labour for trespassing and "hostile acts".
Pyongyang dexterously used their detention as a negotiating card with Washington amid a standoff over its nuclear programs.
North Korea waited four days before announcing on March 21 that they had been detained.
Former President Bill Clinton visited Pyongyang in August to bring them home. That trip, which included a meeting between Clinton and Kim Jong Il, led to the first high-level talks between the two countries earlier this month since US President Barack Obama took office.
North Korea watchers in Seoul, however, said Park's case was unlikely to develop in the same way.