The U.S. national crossed over so quickly that South Korean and American soldiers didn't have time to stop them, according to local media.

A U.S. service member crossed into North Korea and was believed to be in the custody of the country’s forces, a U.S. official told CBS News on Tuesday. It was the United Nations Command, which operates the Joint Security Area within the demilitarized zone between North and South Korea, that first said an unidentified U.S. national had crossed the border.