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

"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.

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.

But "at this time, I don't think very much is known and I don't think that we have successfully made contact with the North Korean authorities."

Wormuth said King "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."