U.S. soldier believed to be in North Korean custody after unauthorized border crossing, officials say-InfoExpress

A U.S. soldier crossed into North Korea "willfully and without authorization" and was believed to be in the custody of the country's forces, U.S. and international officials said Tuesday. The United Nations Command, which operates the Joint Security Area within the Demilitarized Zone between North and South Korea, first confirmed that an unidentified U.S. national had crossed the border.

U.S. officials tell CBS News the soldier in question is Private 2nd Class Travis King, who had served time at a military detention facility in South Korea and was released about a week ago to officials at U.S. Army Garrison Humphreys, the hub of U.S. forces in the country.

King was escorted to the airport in Incheon, near Seoul, for a flight back to the U.S. to be separated from the Army, the U.S. officials said. He parted ways with his escort at Customs and then didn't board the plane. After going through airport security, he somehow returned and managed to join a border tour group before crossing into North Korea, the officials said.

King had served about two months in a South Korean jail on assault charges, a South Korean official told the AFP news agency. According to South Korean media, he was accused of kicking the door of a police patrol car and shouting expletives at officers.

King has served in the U.S. Army since January 2021, Army spokesperson Bryce Dubee told CBS News.

At the Pentagon, Defense Secretary Lloyd Austin confirmed a service member "willfully and without authorization" crossed the Military Demarcation Line separating the two countries during a tour but didn't identify him.

"We believe that he is in DPRK custody," Austin said during a news conference, using an acronym for North Korea's formal name, the Democratic People's Republic of Korea, "and so we're closely monitoring and investigating the situation and working to notify the soldier's next of kin and engaging to address this incident."

"In terms of my concerns, I'm absolutely foremost concerned about the welfare of our troop, and so we will remain focused on this," Austin said.

Earlier, U.N. Command said it was "working with our KPA [North Korean army] counterparts to resolve this incident."

Local media said the man, who was visiting the Military Demarcation Line at Panmunjom with a civilian tour group, crossed the border at 3:27 p.m. local time (2:27 a.m. Eastern).

A 2017 file photo shows a South Korean soldier standing guard before North Korea's Panmon Hall (rear C) at the military demarcation line separating North and South Korea, at Panmunjom, in the Joint Security Area (JSA) of the Demilitarized Zone (DMZ). ED JONES/AFP/Getty

A person who said they witnessed the event and was part of the same tour group told CBS News they had just visited one of the buildings at the site when "this man gives out a loud 'ha ha ha,' and just runs in between some buildings."

The witness said military personnel reacted within seconds to the man's actions, but at first, there was confusion.

"I thought it was a bad joke at first, but when he didn't come back, I realized it wasn't a joke, and then everybody reacted and things got crazy."

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The witness said there were no North Korean soldiers visible where the man ran, and that they were told there haven't been since the coronavirus pandemic, when the North attempted to completely seal its borders to the outside world.

They said after the man ran across the border, the tour group was rushed back to the Freedom House for everyone to give statements and then taken to their bus.

"I'm telling you this because it actually hit me quite hard," the witness said. "It was on the way back in the bus, and we got to one of the checkpoints.... Someone said we were 43 going in and 42 coming back."

They said the group had initially boarded the bus for the tour in Seoul and had been required to provide identification in advance.

Weapons and gear of the DMZ: South Korea's last defense 25 photos

The Demilitarized Zone that separates North and South Korea is one of the most heavily militarized borders in the world. The two Koreas remain technically at war, as fighting in the Korean War ended in 1953 with an armistice agreement, not a full peace treaty. The United States has maintained a large troop presence in South Korea ever since.

Tension between the isolated North Korean regime of Kim Jong Un and the U.S., along with America's key Asian allies South Korea and Japan, has soared over the last decade. After a false start at diplomacy under former President Donald Trump, the North has carried out a steady litany of missile and rocket tests — most of which are condemned by the West as violations of United Nations Security Council resolutions.

U.S. officials have made it clear they expect North Korea to carry out a new test of a nuclear weapon at any point, which would be a significant escalation of what the West considers Kim's provocations.

North Korea fires two short-range ballistic missiles, White House announces new sanctions 04:37

North Korea, on the other hand, regularly warns the U.S. against holding joint military exercises with South Korea, which it claims are rehearsals for an invasion.

U.S. nuclear submarine visits South Korea

The border incident on Tuesday came as the U.S. military confirmed the arrival in a South Korean port of the nuclear-armed submarine USS Kentucky. The visit was the first by a U.S. nuclear submarine to South Korea in four decades, and while it was announced in advance, North Korea was likely to claim it as another American provocation.

South Korea's Defense Minister Lee Jong-Sup said the sub's visit highlighted Washington's commitment to the "extended deterrence" of the threat posed by North Korea.

The U.S. Navy's Ohio class, nuclear-armed ballistic missile submarine USS Kentucky, seen here in a file photo, can carry up to 24 Trident II D-5 ballistic missiles, each with multiple independently-targeted warheads. PO1 Amanda Gray/U.S. Navy

The nuclear-armed sub sitting in the port of Busan "shows the allies' overwhelming capability and posture against North Korea," Lee said.

About a week ago, Kim Jong Un's powerful sister and top adviser Kim Yo Jong warned the U.S. that any perceived reinforcement of the American commitment to defend South Korea would prompt the North to "go farther away from the negotiating table."

She said North Korea was ready "for resolutely countering any acts of violating its sovereignty and territorial integrity," and warned the U.S. to "stop its foolish act of provoking (the North) even by imperiling its security."

Who are the most recent Americans detained in North Korea?

Three men, Kim Dong Chul, Kim Hak Song and Tony Kim, were released in May 2018 from detention in North Korea following a meeting between then-Secretary of State Mike Pompeo and North Korea's Kim. Later that year, Bruce Byron Lowrance was expelled from the country.

A year before, American college student Otto Warmbier was released by North Korea after he had a "severe neurological injury." He died shortly after his release.

— CBS News' David Martin, Eleanor Watson, Emmet Lyons, Tucker Reals and Alex Sundby contributed to this report.