North Korea said Tuesday it has detained an American man who illegally entered the country last week, following reports that a 28-year-old missionary from Arizona crossed into the communist country to draw attention to its human rights record.

The North's official Korean Central News Agency said in a brief dispatch that the American was detained and under investigation after illegally entering through the North Korea-China border last Thursday. It didn't identify the American man.

However, the report comes as South Korean activists say 28-year-old American missionary Robert Park slipped across the frozen Tumen River into North Korea from China last week in an attempt to call attention to the reclusive country's human rights conditions and ask its leader to step down.

The activists had said Park entered North Korea on Friday.

Jo Sung-rae of the Seoul-based activist group Pax Koreana said Tuesday that North Korean border guards apparently detained the missionary as soon as he walked into the communist nation.

Park was carrying letters calling on North Korean leader Kim Jong Il to shut down the country's political prison camps and step down from power, Jo said.

The activist quoted one of two people who guided Park as saying that he "heard people talking loudly when Robert arrived there," and adding that he tought "they were border guards and Robert was taken into custody immediately."

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."

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