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 U.S. diplomats — after activists say he was likely detained while entering the totalitarian state on a faith-fueled 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 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.

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 fervor 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 Rev. 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 and said he was willing to lay down his life.

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

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

Park apparently wanted to appeal directly to Kim.

In one of two letters Park was said to be carrying, he asked that Kim 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. In another, he called for Kim to quit.

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 say it will likely see him as a nuisance and may deport him.

Lee Sang-hyun of the Sejong Institute, a private think tank devoted to security issues, said that with North Korea and the United States now engaged in dialogue, Pyongyang is unlikely to hold Park for long.

"I think there is little chance of the North using him as a negotiating card as it did in the case of the two journalists," he said, referring to reporters Laura Ling and Euna Lee, who worked for former U.S. Vice President Al Gore's Current TV.

They were released from more than four months of captivity in August after a visit to Pyongyang by former President Bill Clinton. Their detention came amid a rupture in dialogue over ending North Korea's nuclear programs.

Now, though, the two sides are talking again after a visit to North Korea earlier this month by President Barack Obama's special envoy for the country.

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 Parks say they are in daily contact with the State Department. The U.S. Embassy in Beijing said that the U.S. consulate in Shenyang in northeastern China is talking with Chinese authorities to try to confirm Park's whereabouts.

Jo, the activist, had initially planned to offer photographic evidence of Park's entry, but said Monday that one of the defectors who videotaped it was demanding money to hand over the film.

Park was born in Los Angeles, but spent much of his childhood in Tucson, where he stayed after his parents moved to the San Diego area about four years ago.

They say he has devoted himself to church work since 2000, and made regular Saturday runs to the slums of Nogales, Mexico, where he distributed food and did missionary work.

He planned to visit South Korea for about a month in June 2008 but ended up staying because he was deeply moved by the poverty of North Koreans, his mother said.

"He felt their pain so much," she said.

Peters, of Helping Hands Korea, said that while his commitment is unquestionable, he wonders whether Park truly understands the gravity of confronting the country head on.

"I don't think he has a real appreciation for the absolute brutality of that regime," he said.

But at Palomar Korean Church in the San Diego suburb of San Marcos, where his parents worship, the Rev. Madison Shockley told about 100 people gathered for a candlelight vigil Sunday that Park's action was justified.

"Robert is doing what God has called him to do," Shockley said. "We call this speaking truth to power."