The soldier is believed to be in North Korean custody, the US official told AFP on condition of anonymity.

The United Nations Command said earlier that an American had crossed into North Korea without authorization while on a tour of the Joint Security Area north of Seoul, but did not specify that he was a member of the US military.

CBS News reported that the soldier was a low-ranking member of the US Army who was being taken to the United States for disciplinary reasons.

He managed to come back and join a tour group after going through airport security, the broadcaster said, citing US officials.

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.

Soldiers from both sides face off at the Joint Security Area, which is overseen by the United Nations Command.

It is a popular tourist destination and hundreds of visitors tour the area on the South Korean side every day.