The American individual, whom U.N. officials did not identify by name, was visiting the Joint Security Area that separates North and South Korea while on an “orientation tour,” the United Nations said. He then crossed the military demarcation line that runs through the demilitarized zone (DMZ) into North Korea, where he was detained, it added.

The Joint Security Area is a 800-meter-wide bubble within the demilitarized zone between North and South Korea where diplomatic discussions can take place. The border area, one of the most fortified in the world, is overseen by the United Nations.

According to the United Nations Command, Korean nationals and foreign tourists are able to visit more than a dozen “Education and Orientation Program sites” located within the demilitarized zone, where visitors can learn about the history of the Korean War and subsequent armistice agreement. It was not immediately clear whether the detained U.S. national was visiting one of those sites.

The State Department strongly advises American citizens not to travel to North Korea due “to the continuing serious risk of arrest and long-term detention of U.S. nationals.” The U.S. government is unable to provide emergency services to its citizens in the country, the State Department notes in its guidance, because no diplomatic or consular relations exist between Washington and Pyongyang.

The U.S. State Department did not immediately respond to a request for more information early Tuesday.

This is a breaking news story and will be updated.