SEOUL, South Korea - An American Christian missionary slipped into isolated North Korea on Christmas Day, shouting that he brought God's love and carrying a letter urging leader Kim Jong Il to step down and free all political prisoners, an activist said yesterday.

Robert Park, 28, of Tucson, Ariz., crossed a poorly guarded stretch of the frozen Tumen River that separates North Korea from China, according to a member of the Seoul-based group Pax Koreana, which promotes human rights in the North. Two other activists apparently watched and filmed the entry.

"I am an American citizen. I brought God's love. God loves you, and God bless you," Park reportedly said in fluent Korean as he crossed over near the northeastern city of Hoeryong, according to the activist, citing the two who witnessed the scene.

Pax Koreana planned to release the footage today in Seoul, he said. He spoke on condition of anonymity because of the sensitivity of the situation.

No information has emerged about what happened next to Park, who is of Korean descent. The communist country's state-run media were silent. The State Department and the U.S. Embassy in Beijing said they were aware of the incident but had no details.

The illegal entry could complicate Washington's efforts to coax North Korea back to negotiations aimed at its nuclear disarmament. Park's crossing comes just months after the country freed two U.S. journalists, who were arrested along the Tumen and sentenced to 12 years of hard labor for trespassing and "hostile acts." The two women were released in August to former President Bill Clinton. North Korea and the United States have no diplomatic relations.

Park carried a letter to Kim Jong Il calling for major changes to his totalitarian regime, according to the activist from Pax Koreana.

"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 the conservative group's Web site. "Please close down all concentration camps and release all political prisoners today."

North Korea holds about 154,000 political prisoners in six large camps, according to South Korean estimates. The North has long been regarded as having one of the world's worst records for human rights, but it denies the existence of prison camps.

North Korea's criminal code punishes illegal entry with up to three years in prison.