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.
In an update on Tuesday evening, US defence secretary Lloyd Austin confirmed the details of the case and said authorities are working to notify the soldier’s next of kin.
Mr Austin said his “foremost concern” was the troop’s welfare.
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.
The US bans its citizens from entering North Korea – the notorious totalitarian state run by Kim Jong Un where millions live in hunger and poverty.
Some Americans have been accused of spying in recent years after entering the country from China, with careful diplomacy needed to get them released.
Tuesday’s incident happened as the US sent a nuclear-armed submarine to South Korea for the first time in decades.
It’s seen as a deterrence move following numerous North Korea missile tests, including one last week that flew for 74 minutes – the longest yet.
That launch followed recent complaints from Pyongyang that US spy planes had violated its airspace.