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NEW: U.S. Embassy says it's aware of reports the man was trying to swim to North Korea

A South Korean official says not immediately clear if the man was attempting to cross the border

He was caught on the bank of the Han River where it divides the two Koreas

Seoul, South Korea CNN  — 

South Korean marines detained an American man on the bank of a river bordering North Korea late Tuesday, according to a South Korean Defense Ministry official and a senior U.S. State Department official.

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 U.S. citizen was being interrogated Wednesday, the defense ministry official said, adding that it was not immediately known whether the American was trying to cross into North Korea.

The U.S. Embassy in Seoul said it was “aware of the reports that a U.S. citizen has been detained attempting to swim from South Korea to North Korea.”

The Embassy said it had been in contact with South Korean authorities but didn’t have any additional information to share.

Americans are allowed to travel to North Korea, usually arriving by plane from Beijing. But it’s forbidden to cross from South Korea into North Korea. The U.S. State Department warns citizens against all travel to the authoritarian state led by Kim Jong Un.

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.

Fast facts on detained Americans

CNN’s K.J. Kwon reported from Seoul, and Jethro Mullen from Hong Kong. CNN’s Paula Hancocks, Elise Labott and Josh Levs contributed to this report.