A US soldier facing disciplinary action has dashed across the inter-Korean border into North Korea and is believed to be in North Korean custody, US officials say.

US Defence Secretary Lloyd Austin expressed concern for the soldier, who the US military in South Korea said was on an orientation tour of Joint Security Area between the Koreas and “wilfully and without authorisation crossed the Military Demarcation Line into the Democratic People’s Republic of Korea“.

“There’s a lot that we’re still trying to learn,” Mr Austin said on Wednesday (Australian time).

“We believe that he is in [North Korean] custody and so we’re closely monitoring and investigating the situation and working to notify the soldier’s next of kin.”

Colonel Isaac Taylor, spokesman for US Forces Korea, said the military was “working with our KPA counterparts to resolve this incident,” referring to North Korea’s People’s Army.

US media has named the man has Private Travis King, a calvary scout who joined the military in 2021 and had been on assignment in South Korea.

Army spokesperson Bryce Dubee has told US media that Private King had earned several awards during his service, including National Defence Service Medal, the Korean Defence Service Medal and Overseas Service Ribbon.

CBS News said that before the incident Private King was being escorted back to the US for disciplinary reasons. After going through airport security, he somehow returned and managed to join the border tour.

It said a person who said they witnessed the event and was part of the same tour group told CBS News they had just visited one of the buildings at the site when “this man gives out a loud ‘ha ha ha’ and just runs in between some buildings”.

CBS cited the witness as saying that military personnel reacted within seconds but initially there was confusion.

“I thought it was a bad joke at first. But when he didn’t come back, I realised it wasn’t a joke, and then everybody reacted and things got crazy,” the witness said.

According to CBS, the witness said there were no North Korean soldiers visible when Private King ran, and that the group was told that none had been seen since the coronavirus pandemic when North Korea sealed its borders.

The crossing comes at a sensitive time amid high tensions on the Korean peninsula, with the arrival of a US nuclear-armed ballistic missile submarine in South Korea for a rare visit in a warning to North Korea over its own military activities.

North Korea has been testing increasingly powerful missiles capable of carrying nuclear warheads, including a new solid-fuel intercontinental ballistic missile launched last week.

The incident happened during a tour to the Joint Security Area on the demilitarised zone border that has separated the two Koreas since the end of the 1950-53 Korean War.

Movement along the 250-kilometre boundary line between South and North Korea is strictly monitored by authorised guides, with visitors instructed to stand or walk only in certain areas.

Former US president Donald Trump made history by stepping over a small slab of concrete from South Korea into North Korea in 2019 as part of failed efforts to advance peace talks or curb Pyongyang’s weapons program.

When Private King vanished this week, he was with a group of visitors, including civilians, in the Panmunjom truce village when he suddenly bolted over the brick line marking the border, Donga and the Chosun Ilbo newspapers reported, citing South Korean army sources.

He is believed to be the first US soldier to cross into North Korea since 1982.

The US State Department tells US citizens not to enter the reclusive nation “due to the continuing serious risk of arrest and long-term detention of US nationals”.

The ban was implemented after US college student Otto Warmbier was detained by North Korean authorities while on a tour of the country in 2015.

He died in 2017, days after he was released from North Korea and returned to the US in a coma.

-with AAP