Iran Press/ America: The United Nations Command, which operates the Demilitarised Zone and joint security area (JSA), said the man did not have authorization.
"We believe he is currently in DPRK custody and are working with our KPA counterparts to resolve this incident," it said.
BBC News has contacted the US Department of State for comment.
The man is believed to be the only US citizen in North Korean custody.
207
Read more:
North Korea's UN envoy defends missile launch
North Korea fires ballistic missile toward East Sea