South Korean marines detained an American man on the bank of a river bordering North Korea late Tuesday, a South Korean Defense Ministry official told CNN.
The marines were on a regular patrol mission west of Seoul when they caught the man by the Han River in an area where it divides North and South Korea.
The official said Wednesday that the U.S. citizen was being interrogated by "relevant South Korean officials," and that it was not immediately whether he had been trying to cross into North Korea.
The defense official asked not to be identified, citing South Korean media practices. He also declined to give any further information about the American man's case.
Americans are allowed to travel to North Korea, notably by plane from Beijing, although the U.S. State Department warns citizens against all travel to the authoritarian state led by Kim Jong Un. It's forbidden, however, to cross from South Korea into North Korea.
Three U.S. citizens are currently in detention in North Korea: Kenneth Bae, Matthew Todd Miller and Jeffrey Fowle.
A North Korean court on Sunday sentenced Miller to six years hard labor for committing "acts hostile" to North Korea, although the circumstances surrounding his alleged crime remain murky.
Bae, a Korean-American missionary, is serving a 15-year sentence for allegedly trying to bring down the North Korean government. Fowle, who was arrested in June while traveling as a tourist, is still awaiting trial.