WASHINGTON - The United States on Thursday voiced mounting concern over army Private Travis King, who dashed into North Korea two days ago, saying Pyongyang had a history of mistreating captured Americans.

US Army Secretary Christine Wormuth, in her first public comments on the case, said Washington was fully mobilised in trying to contact Pyongyang, including through United Nations communications channels.

But North Korea has yet to offer any response, officials said.

“I worry about him, frankly,” Ms Wormuth told the Aspen Security Forum in Colorado. She cited the case of Mr Otto Warmbier, a US college student who was imprisoned in North Korea for 17 months before dying shortly after he was returned to the US in a coma in 2017.

“I worry about how they may treat him. So, (we) want to get him back.”

At the White House, National Security Council spokesman John Kirby also expressed concern: “This is not a country that is known for humane treatment of Americans, or frankly, anybody else for that matter.”

American officials remained stumped about why Pte King ran across the border into North Korea. But Ms Wormuth acknowledged he was likely worried about facing further disciplinary action from the army upon his return home to the US.

She said she was not aware of any information demonstrating the 23-year-old was a North Korea sympathiser, and the Pentagon played down suggestions he might present an intelligence liability.

Pentagon spokesman Sabrina Singh said the US Army’s counter-intelligence office and US forces in South Korea were carrying out an investigation into what drove Pte King to make such a puzzling decision.

Still alive?

Ms Singh declined to directly respond to a question about whether the Pentagon believed Pte King was still alive. She said the US military could not offer any information at all about his status.

“We don’t know his condition. We don’t know where he’s being held. We don’t know the status of his health,” Ms Singh said, describing his formal status in the military as AWOL, or absent without leave.

North Korea has remained silent about Pte King and US 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 US nationals, has not commented on the incident so far.

Speaking in Japan, US special envoy for North Korea Sung Kim said the US was “working very hard” to determine Pte King’s status and well-being and is actively engaged in ensuring his safety and return. Mr Kim did not provide any details.

Pte 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.

Pte 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 US military for a commercial flight to Dallas, Texas, according to US 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.

Ms Wormuth said Pte 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 US and face the consequences in the army,” she said. “I’m sure that he was grappling with that.”

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

Asked whether Pte King might have sympathised with North Korea, Ms Wormuth said: “I don’t think we have any information that points to that clearly.”

The Pentagon said it was not aware of any changes to freedom of movement to roughly 28,500 US forces in South Korea.

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 US 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 US 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. REUTERS