SEOUL, South Korea - 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.

It was unclear Saturday if he was in custody.

Park is a missionary from Tucson, Ariz., 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.

“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.