South Korea (AP):

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

Now, the 28-year-old Korean-American missionary is himself the focus of prayers and a search by United States diplomats after activists say it is likely he was detained while entering the totalitarian state on a faith-fuelled mission to liberate its people.

Park walked across the frozen Tumen River from China on Christmas Day, 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 organisation 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.

North Korean state media, which waited four days to announce Pyongyang was holding two American journalists after they were detained along the border in March, has so far reported nothing about any intrusion.

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 fervour and passion for the cause. Tim Peters, founder and director of Helping Hands Korea, a Christian charity group supporting North Korean refugees, said Park is infused with "a very, very serious commitment to prayer".

The Reverend John Benson, the pastor of Life in Christ Community Church in Park's hometown of Tucson, Arizona, said Park's devotion is exceptional.

"You have to understand that for this guy, when it comes to the Lord, he's very, very serious," said Benson, who led weekend services to pray for his safe return. "Unusually serious."

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