By Phil Stewart and Idrees Ali

WASHINGTON The U.S. Army on Thursday expressed deep concern for how North Korea might treat Private Travis King and said Washington was fully mobilized in its efforts to reach out to Pyongyang, including by using United Nations communications channels.

Army Secretary Christine Wormuth, in her first public comments on the case, said it remained unclear why King ran across the border into North Korea but acknowledged he was likely concerned about facing further disciplinary action from the Army upon his return home to the United States.

She said she was not aware of any information demonstrating the 23-year-old was a North Korea sympathizer.

"I worry about him, frankly," Wormuth told the Aspen Security Forum in Colorado, citing the case of Otto Warmbier, a U.S. college student who was imprisoned in North Korea for 17 months before dying in 2017 shortly after he was returned to the United States in a coma. "I worry about how they may treat him. So, (we) want to get him back."

North Korea has remained silent about King and U.S. officials say Pyongyang has not responded to communication from the American military about the soldier. North Korea's state media, which has in the past reported on the detention of U.S. nationals, has not commented on the incident so far.

Speaking in Japan, U.S. special envoy for North Korea Sung Kim said the United States was "working very hard" to determine King's status and well-being and is actively engaged in ensuring his safety and return. Kim did not provide any details.

White House spokesperson Olivia Dalton said on Thursday that the U.S. government has contacted the North Korean military through "multiple channels" and is still trying to learn King's whereabouts and condition.

King was on a civilian tour of the Panmunjom truce village on Tuesday when he dashed across the Military Demarcation Line that has separated the two Koreas since the Korean War ended with an armistice in 1953.

King had been fined for assault while stationed in South Korea and had been detained for more than a month before being escorted to Incheon International Airport by the U.S. military for a commercial flight to Dallas, Texas, according to U.S. officials.

Once past security checks, he told airline staff at the departure gate he had lost his passport and returned to the terminal, an airport official told Reuters on condition of anonymity.

Wormuth said King "may not have been thinking clearly, frankly."

"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," she said. "I'm sure that he was grappling with that."

North Korea and the United States have no formal diplomatic ties following years of international sanctions imposed on the reclusive state for its nuclear arms and missile programs that have drawn frequent U.N. condemnation.

Asked whether King might have sympathized with North Korea, Wormuth said: "I don't think we have any information that points to that clearly." Tensions are heightened on the Korean peninsula. The North has been conducting ballistic missile tests, the latest coinciding with the arrival in South Korea of a U.S. nuclear-armed ballistic missile submarine for the first time since the 1980s.

Last week, North Korea launched its newest solid-fuel intercontinental ballistic missile (ICBM) which it said had the longest flight time ever.

On Monday, North Korea's Kim Yo Jong, sister of leader Kim Jong Un and a powerful ruling party official, said the United States should stop its "foolish act" of provoking North Korea and said it was putting its security at risk.

She made her comments after White House national security adviser Jake Sullivan said Washington remained concerned that North Korea would carry out another ICBM test.

(Reporting by Phil Stewart and Idrees Ali; Editing by Jonathan Oatis and Daniel Wallis)