SEOUL, South Korea (AP) — An American has crossed the heavily fortified border from South Korea into North Korea, the American-led U.N. Command overseeing the area said Tuesday.

The U.N. Command tweeted that the U.S. citizen was on a tour to the Korean border village of Panmunjom and crossed the border into the North without authorization.

It said he is currently in North Korean custody and that the U.N. Command is working with its North Korean counterparts to resolve the incident.

It gave no further details on who the person is or why he crossed the border.

Cases of Americans or South Koreans defecting to North Korea are rare, though more than 30,000 North Koreans have fled to South Korea to avoid political oppression and economic difficulties since the end of the 1950-53 Korean War.

Panmunjom, located inside the 248-kilometer (154-mile) -long Demilitarized Zone, was created at the close of the Korean War. Bloodshed and gunfire have occasionally occurred there, but it has also been a venue for numerous talks and a popular tourist spot.

The area is jointly overseen by the U.N. Command and North Korea.

In November 2017, North Korean soldiers fired 40 rounds as one of their colleagues raced toward freedom. The soldier was hit five times before he was found beneath a pile of leaves on the southern side of Panmunjom. He survived and is now in South Korea.

No civilians live at Panmunjom.