A U.S. national on a tour to the joint security area between North and South Korea crossed the Military Demarcation Line without authorization and is now believed to be in detained in North Korea, the U.N. Command announced Tuesday.

"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)," the peacekeeping force said in a tweet. "We believe he is currently in DPRK custody and are working with our Korean People's Army counterparts to resolve this incident."

The identity of the individual was not immediately available.

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, according to The Associated Press.

The Korean border village of Panmunjom, located inside the 154-mile-long Demilitarized Zone, was created at the close of the Korean War. The area is jointly overseen by the U.N. Command and North Korea.

The Associated Press contributed to this report.

This is a developing story. Please check back for updates.