An American has crossed the heavily fortified border from South Korea into North Korea, officials confirmed on Tuesday, amid high tensions over Pyongyang’s nuclear program.

The U.S.-led U.N. Command overseeing the demilitarized zone, or DMZ, confirmed that the U.S. citizen was on tour around the Korean border village of Panmunjom before crossing the border into North Korea without proper authorization. There were also reports saying that the man who crossed the border is an American soldier.

“A U.S. National on a JSA orientation tour crossed, without authorization, the Military Demarcation Line into the Democratic People’s Republic of Korea (DPRK). We believe he is currently in DPRK custody and are working with our KPA counterparts to resolve this incident,” the U.N. Command said in a Twitter post on Tuesday.

The post gave no further details on who the person is or why he crossed the border. North Korean state-run media didn’t immediately comment on the border incident.

Panmunjom, located inside the 154-mile-long DMZ, is jointly overseen by the U.N. Command and North Korea since its creation 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.

Known for its blue huts straddling concrete slabs that form a military demarcation line, Panmunjom has drawn visitors on both sides, who want to see what the Cold War’s last frontier looks like. No civilians live at Panmunjom.

Tours to the southern side of the village reportedly drew around 100,000 visitors a year before the pandemic, when South Korea restricted gatherings to slow the spread of COVID-19. The tours fully resumed last year.

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.

Tourists who partake in the JSA tour are kept about 60 feet from the DMZ line, which former President Donald Trump famously crossed in 2019 to meet North Korean dictator Kim Jong Un. There are generally guards on both sides of the border during the tour.

The border incident comes as a U.S. Navy nuclear-capable submarine on made a port of call in South Korea for the first time in decades.

The Associated Press contributed to this report. This is a breaking news story; check back for more updates.