An American soldier facing disciplinary action fled across the inter-Korean border into North Korea on Tuesday and was believed to be in North Korean custody, US officials said, creating a fresh crisis for Washington in its dealings with the nuclear-armed state.

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

“There’s a lot that we’re still trying to learn,” Austin told a news briefing.

“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, spokesperson 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.

Reuters could not immediately confirm the identity of the soldier, but two US officials, who spoke on condition of anonymity, said he had been due to face disciplinary action by the US military.

South Korea’s Dong-a Ilbo daily, citing South Korea’s army, identified the person as Travis King, a US army soldier with the rank of private second class.

The newspaper later deleted the name.

CBS News said that before the incident the soldier was being escorted back to the United States for disciplinary reasons, but after going through airport security 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 to the man’s actions, 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,” it quoted the witness as saying.

According to CBS, the witness said there were no North Korean soldiers visible where the man ran, and that the group has been told there hadn’t been since the coronavirus pandemic, when North Korea sought to seal 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 White House, the US State Department, the Pentagon and North Korea’s mission to the United Nations in New York did not immediately respond to requests for comment.

The Joint Security Area is in the demilitarised zone border that has separated the two Koreas since the end of the 1950-53 Korean War.

The man was with a group of visitors, including civilians, to the Panmunjom truce village when he suddenly bolted over the brick line marking the border, Donga and the Chosun Ilbo daily newspapers reported, citing South Korean army sources.

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

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 United States in a coma.