North and South Korea remain technically at war since the 1950-1953 Korean War ended with an armistice rather than a peace treaty, with a Demilitarized Zone running along their border.

"A US service member on a JSA orientation tour willfully and without authorization crossed the Military Demarcation Line" into North Korea, spokesman Colonel Isaac Taylor said in a statement, referring to the Joint Security Area north of Seoul.

A US official previously told AFP on condition of anonymity that the American national was a soldier who is now believed to be in North Korean custody.

CBS News reported that the soldier was a low-ranking member of the US Army who was being escorted home to the United States for disciplinary reasons, but somehow managed to leave the airport and join the tour group.

Troops from both North and South Korea face off at the Joint Security Area, a popular tourist destination that is toured by hundreds of visitors on the South Korean side every day.