An American missionary is believed to be in North Korean custody after he illegally entered the communist nation on Christmas Day in an attempt to call attention to the reclusive country's human rights conditions, an activist said Sunday.

Robert Park, 28, slipped across the frozen Tumen River into the North from China on Friday. There has been no word from him since, but an activist who knows Park said he was likely arrested quickly by border guards or police in the authoritarian state.

"We haven't heard anything about him since he went there," said the activist, a member of the Seoul-based group Pax Koreana, which promotes human rights in the North. "But it's North Korea. We presume he has been arrested there."

Analysts said North Korea would most likely expel the Christian missionary instead of holding him as a negotiating card because that would serve his purpose of highlighting the regime's human rights abuses.

The communist country's state-run media did not mention Park's case Sunday. The U.S. State Department and the U.S. Embassy in Beijing said they were aware of the incident but had no details.

Park, who is of Korean descent, was carrying a letter urging leader Kim Jong Il to step down and free all political prisoners, the activist said.

"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 Pax Koreana activist.

Two other activists filmed Park cross into the North, the activist told The Associated Press, speaking on condition of anonymity because of the sensitivity of the situation.

Park's crossing comes just months after North Korea freed two U.S. journalists who had been arrested in March and sentenced to 12 years of hard labor for trespassing and "hostile acts." Pyongyang dexterously used their detention as a negotiating card with Washington amid a standoff over its nuclear programs.

North Korea waited four days before announcing on March 21 that they had been detained.

Former President Bill Clinton visited Pyongyang in August to bring them home. That trip, which included a meeting between Clinton and Kim Jong Il, led to the first high-level talks between the two countries earlier this month since President Barack Obama took office.

North Korea watchers in Seoul, however, said Park's case was unlikely to develop in the same way.

"I think it will end up an isolated episode," said Kim Yong-hyun, an expert on North Korea at Seoul's Dongguk University. "North Korea knows that it would be serving the purpose of the activist and highlight its human rights problems if it keeps holding him like it did the journalists."