A U.S. soldier stationed in South Korea crossed the heavily armed border into North Korea without authorization and is believed to be in North Korean custody, officials said Tuesday.

White House spokeswoman Karine Jean-Pierre and other officials said the soldier went "willfully" across the military demarcation line into North Korea in the afternoon local time.

File photo taken in July 2019 shows the truce village of Panmunjeom on the South Korean side. (Kyodo)

Defense Secretary Lloyd Austin also told a press conference, "We believe that he is in (North Korea's) custody. And so we're closely monitoring and investigating the situation and working to notify the soldier's next of kin and engaging to address this incident."

State Department spokesman Matthew Miller said, "As always, the safety of and security of any American overseas remains the top priority," adding that the Pentagon has already reached out to North Korea.

The soldier, identified as Travis King, holds the rank of private second class in the Army, according to Army spokesman Bryce Dubee.

Quoting U.S. officials, Reuters news agency reported the soldier had been due to face disciplinary action by the U.S. military.

The American was on a tour of the inter-Korean truce village of Panmunjeom before entering North Korea, according to the U.S.-led U.N. Command, which said that it is working to resolve the incident with the North Korean military.

The United States and North Korea have no diplomatic relations.

The United States is expected to request that North Korea release the American. Some analysts say Pyongyang may use the man as a bargaining chip in future negotiations with Washington.

The U.N. Command oversees activities in the Demilitarized Zone dividing the two Koreas.