SEOUL, South Korea (AP): 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 Saturday.

Robert Park, 28, 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 Friday near the northeastern city of Hoeryong, according to the activist, citing the two who witnessed the scene. Pax Koreana planned to release the footage Sunday in Seoul, he said. He spoke on condition of anonymity due to 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 was silent. The State Department and the U.S. Embassy in Beijing said they were aware of the incident but had no details.

"The U.S. government places the highest priority on the protection and welfare of American citizens," said State Department spokesman Andrew Laine.

The illegal entry could complicate Washington's efforts to coax North Korea back to negotiations aimed at its nuclear disarmament. Park's crossing also 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." They were released to former President Bill Clinton on a visit to the isolated country in August. North Korea and the United States do not have diplomatic relations.

Park, from Tucson, Arizona, carried a letter to North Korean leader 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 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 denies the existence of prison camps.

The activist said that Park, who he described as not belonging to Pax Koreana, also carried a separate written appeal calling for Kim to immediately step down, noting starvation, torture and deaths in North Korean political prison camps.

North Korea's criminal code punishes illegal entry with up to three years in prison. But that could be the least of the missionary's problems in a country where defectors say dissent is swiftly wiped out and the regime sees all trespassers as potential spies.

Kim wields absolute power in the communist state of 24 million people where he and his late father ? the country's founder Kim Il Sung ? are the object of an intense personality cult.

Demanding Kim step down is "a kind of hostile act," said Koh Yu-hwan, a professor at Seoul's Dongguk University.

Similar cases are extremely rare, but any kind of entry deemed illegal leads to serious consequences ? as the detention of American reporters Laura Ling and Euna Lee earlier this year proved.

In 1996, Evan C. Hunziker was detained for three months after swimming across the Yalu River, also on the Chinese border.

Hunziker, who was 26, said he went there out of curiosity and "to preach the Gospel." Other reports said he got drunk and decided to go for a swim.

He was eventually freed after New Mexico Gov. Bill Richardson, who was then a congressman, negotiated his release.

Analyst Paik Hak-soon of the private Sejong Institute think tank said he doubted the U.S. will be forced to send a special envoy this time as the two countries recently committed to maintaining dialogue on the nuclear issue and may be able to resolve it through existing diplomatic channels.

Still, he thought the incident could hinder warming relations. "North Korea's release of the missionary won't be easy," Paik said.

Park came to South Korea in July and stayed there until leaving for China earlier this week to enter the North, said the Pax Koreana activist. He said Pax Koreana is affiliated with another organization called Freedom and Life For All North Koreans to which Park belongs.

Other activists said Park had become known over the last year in Seoul human rights circles. They suggested that his passion for helping North Koreans may have blinded him to the consequences of his actions.

"I just feel that this was a reckless and misguided adventure," said Tim Peters, founder and director of Helping Hands Korea, a Christian charity group supporting North Korean refugees.

He said Park had a deep and admirable commitment to prayer and the North Korean cause but added he was a "newcomer" to such activism and "out of his depth."

Peter Jung, an official with Justice for North Korea, a Seoul-based advocacy group, said Park frequently met with North Korean defectors to try to learn about the country.

"His unilateral affection, deep attachment and passion were so strong that he was not able to harmonize with the people around him," Jung said.