SEOUL: A Christian missionary from the U.S. has entered North Korea carrying a letter to leader Kim Jong Il in order to call attention to the tens of thousands of political prisoners believed held in the communist state, an activist said Saturday.

Robert Park, a 28-year-old Korean-American, crossed the frozen Tumen River into North Korea from China on Christmas Day to urge Kim to release political prisoners and shut down the "concentration camps" where they are held, said the activist, who spoke on condition of anonymity, citing the issue's sensitivity.

"I am an American citizen. I brought God's love. God loves you and God bless you," Park was quoted by two activists as shouting in Korean as he crossed the North Korean border, according to the activist who spoke to The Associated Press.

North Korea holds some 154,000 political prisoners in six large camps across the country, according South Korean government estimates. The North has long been regarded as having one of the world's worst human rights records, but it rejects outside criticism and denies the existence of prison camps.

Park is a missionary from Tucson, Arizona, according to the activist, who works for Pax Koreana, a conservative Seoul-based group that calls for North Korea to improve its human rights record.

He said Park was last seen by the two other activists who saw him enter North Korea's northeastern city of Hoeryong from the poorly guarded border late Friday afternoon. He added that the crossing was videotaped and the footage would be released Sunday. It was unclear if he was in custody.

North Korean state media did not mention any crossing. Conviction for illegal entry carries up to three years in prison under the country's criminal code.

"We are aware of Robert Park," said Susan Stevenson, a spokeswoman for the U.S. Embassy in Beijing. She said a charitable organization she did not identify notified the State Department in Washington. The embassy is looking into the case but had no details.

"His fate to us is unknown," she said.

Park was carrying a letter to North Korean leader Kim, according to the activist.

"Please open your borders so that we may bring food, provisions, medicine, necessities, and assistance to those who are struggling to survive," said the letter, according to a copy posted on Pax Koreana's Web site. "Please close down all concentration camps and release all political prisoners today."

The activist said Park came to South Korea in July and stayed there until leaving for China earlier this week to enter the North.

"I would not go to North Korea to live. Even if I die, world leaders should really repent for keeping silence" on North Korea, Park said in Seoul before leaving for China, the activist said.

The activist said Pax Koreana is affiliated with another organization called Freedom and Life For All North Koreans, which is a coalition of advocacy groups for North Korean human rights. Park is a member of the broader group, he said. The coalition and other activist groups plan to hold rallies in New York, Tokyo, Seoul and other cities from Sunday to Thursday.

In August, North Korea released two U.S. journalists it had sentenced to 12 years of hard labor for trespassing and "hostile acts." Their release came amid a trip to Pyongyang by former President Bill Clinton aimed at winning their freedom.

Journalists Laura Ling and Euna Lee were captured by North Korean guards near the Tumen River in March while reporting a story on North Korean defectors.

Park's reported entry comes weeks after North Korea held one-on-one talks with the United States and signaled its willingness to return to international negotiations on ending its nuclear weapons programs. Pyongyang said earlier this month it would try to resolve remaining differences with Washington.

In the message Park is carrying, he also demanded Kim Jong Il and other leaders immediately step down, noting alleged starvation of North Koreans and deaths in political prison camps.

Kim wields absolute power in the communist state of 24 million people and allegedly inmates cannot be executed without his knowledge or direction, activists say.