Officials said Thursday, that the United States fears an American soldier held by North Korea may be mistreated by the country, which has yet to respond to inquiries about his fate.

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 Department of Defense said US Army launched an investigation into the incident, but officials made it clear that their first priority was 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 North Korea'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.