SEOUL, South Korea  North Korea said Tuesday that it had detained an American citizen who the government said had illegally entered the reclusive country from China.

The brief dispatch by the North’s official news agency, K.C.N.A., did not identify the person, who it said was under interrogation by the authorities. It said the intruder crossed the river border into North Korea on Thursday.

An American missionary from Tucson entered North Korea from China on Friday. The missionary, Robert Park, 28, had said he intended to persuade the North’s leader, Kim Jong-il, to release political prisoners and that he hoped to get arrested to bring attention to his cause, according to human rights activists in Seoul.

It was not possible to determine whether the person arrested by the North was Mr. Park.

Mr. Park was carrying a Bible and a letter urging the Mr. Kim Jong-il to release all political prisoners from the country’s notorious concentration camps, the activists said.