The incident comes as America deploys a nuclear-armed submarine to the region for the first time in years following more missile tests by Pyongyang.
A US man is believed to be in custody in North Korea after he crossed the border without permission during a tour.
His identity has not been officially confirmed but South Korean newspaper The Dong-a Ilbo reported he is a US Army solider named Travis King.
It happened at the Joint Security Area, part of the 160-mile demilitarised zone separating South and North Korea.
“A US national on a JSA orientation tour crossed, without authorisation, the Military Demarcation Line into the Democratic People’s Republic of Korea (DPRK),” the UN Command tweeted.
“We believe he is currently in DPRK custody and are working with our KPA counterparts to resolve this incident,” it added.
South and North Korean forces face each other at the Joint Security Area and it’s often used for diplomatic meetings between the two countries.
It is also a popular tourist spot.
A spokesman for the US military in South Korea, Colonel Isaac Taylor, said: “We’re still doing some research into this, and everything that happened.”
In 2017, a North Korean soldier was fired at by colleagues as he made a break for freedom across the JSA border.
He was hit five times but survived and now lives in the South.