Private Second Class Travis King was due to return to the United States to face disciplinary consequences after serving jail time in South Korea, but instead left the airport, joined a tour of the border and fled across it.

The US Army has launched an investigation into the incident, the Pentagon said, but officials have made clear that their first priority is securing the soldier's release.

"It makes me very, very concerned that Private King is in the hands of the North Korean authorities... I worry about how they may treat him," Secretary of the Army Christine Wormuth said at the Aspen Security Forum.

She referenced the case of Otto Warmbier, an American who was held for a year-and-a-half before being released in a coma to the United States and dying six days later.

State Department spokesman Matt Miller also cited Pyongyang's history of mistreating detainees.

"We would always have concern given the treatment by North Koreans of past detained individuals and that's one of the reasons why we are reaching out to ask for more information about his well being," Miller said.

"The case continues to be an extremely high priority for the department. Our diplomatic efforts are ongoing," he said.

- Not 'thinking clearly' -

Wormuth said Washington has sought information about King through UN channels, and that various parts of the government are involved in the effort.

"The Department of Defense, the State Department, the White House, we're using UN channels, have been reaching out to (North Korea) to get information about his status and to work with them to try to... bring him back," she said.

Wormuth said the soldier "would have faced additional consequences" had he returned to the United States as planned, though it was unclear if jail time was on the table.

He "had assaulted an individual in South Korea and had been in custody of the South Korean government and was going to come back to the United States and face the consequences in the Army. And I'm sure that he was grappling with that," she said.

"He may not have been thinking clearly, frankly, but we just don't know."

North and South Korea remain technically at war since the 1950-1953 Korean War ended with an armistice rather than a peace treaty, with a Demilitarized Zone running along their border.

King was on a tour of the Joint Security Area -- where soldiers from North and South Korea face off -- when he crossed the border.

Deputy Pentagon Press Secretary Sabrina Singh said King was escorted up to airport security and that "I don't think anyone anticipated that he was going to leave the airport."

US Army counterintelligence is leading an investigation into the incident, which will determine whether or not he is a defector, she said, noting that his current status is absent without leave, or AWOL.

But "our biggest concern about Private King is that we want to bring him home, and we are doing whatever we can... to find a way," Singh said.