A US national has crossed the inter-Korean border into North Korea without authorization while on a tour and is likely to be in the North’s custody, the United Nations Command that oversees the demilitarised zone area at the border said on Tuesday.

The person was taking part in a tour to the Joint Security Area on the demilitarised zone border separating the two Koreas since the end of the 1950-53 Korean War where soldiers from both sides stand guard.

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.

Reuters could not immediately verify the report.

“A US National on a JSA orientation tour crossed, without authorization, the Military Demarcation Line into the Democratic People’s Republic of Korea (DPRK),” the UN Command (UNC) said on Twitter.

“We believe he is currently in DPRK custody and are working with our KPA counterparts to resolve this incident,” it added, referring to North Korea’s People’s Army.

Colonel Isaac Taylor, spokesperson for the US military in South Korea (USFK) and the UN Command, declined to confirm whether the individual was a US Army soldier or a member of USFK, saying he had nothing to add to the UNC statement.

“We’re still doing some research into this, and everything that happened,” he told Reuters.

The White House, the US State Department and the Pentagon did not immediately respond to requests for comment.

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 (ICBM) launched last week.

South Korea’s Defence Ministry said it did not immediately have any information on the border incident.

US State Department travel advisory bans US nationals from entering North Korea “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 United States in a coma.