An American citizen who crossed into North Korea without authorization on Tuesday has been taken into custody by North Korean authorities, the American-led United Nations Command said.

The American national crossed into North Korea during a tour of Panmunjom, or the Joint Security Area, which straddles the inter-Korean border, becoming the latest United States citizen to be detained by the isolated Communist country.

The U.N. Command said in a statement that it was working with the North Korean military “to resolve this incident” but gave no further information.

Both the U.N. Command and the North Korean People’s Army keep duty officers at Panmunjom, the sole point of contact on the 155-mile-long Demilitarized Zone that separates the two Koreas.

The U.N. Command allows tour groups in the Joint Security Area, which was created as part of the 1953 armistice that halted the Korean War 70 years ago next week. Tourists can visit the area from South Korea while unarmed soldiers trail closely behind them.

The man detained on Tuesday was the first known American held in North Korean custody since​ Bruce Byron Lowrance​ was detained for a month after illegally entering the country from China in 2018.​

The American student Otto F. Warmbier was arrested in Pyongyang in 2016 for allegedly trying to steal a propaganda poster from the wall of his hotel. Mr. Warmbier was sentenced to 15 years in prison. After being held for 17 months in North Korea, Mr. Warmbier, then 20, was flown from Pyongyang to Ohio, his home state, in a coma in June 2017.

He died a week later, which outraged his family.

This is a developing story.