Pyongyang, North Korea - The US national taken into North Korean custody after crossing the heavily fortified border without authorization is believed to be a soldier who served around two months in a South Korean jail on assault charges, per authorities.

The soldier – identified by the US military as Travis King, a private second class who has been in the army since 2021 – crossed the border "willfully and without authorization," US Forces Korea spokesman Colonel Isaac Taylor said.

The United Nations Command said he had been on a Joint Security Area (JSA) orientation tour, adding he was believed to be in North Korean custody and that it was working with Pyongyang's military to "resolve this incident".

"King was released on July 10 after serving around two months in a South Korean prison for assault charges," a Seoul official told AFP.

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South Korean police told AFP that King had been investigated for assault in September 2022, but was not detained at the time.

CBS News, citing US officials, reported that the low-ranking soldier was being escorted home to the US for disciplinary reasons, but managed to leave the airport and join the tour group.

Pentagon chief Lloyd Austin told journalists that Washington was "closely monitoring and investigating the situation".

North and South Korea remain technically at war as the 1950-1953 Korean War ended with an armistice rather than a peace treaty, with a Demilitarized Zone running along the border.

An eyewitness who said they were on the same JSA tour told CBS News the group had 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".

"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," they said.

Hours later, North Korea fired two short-range ballistic missiles into the sea, according to the South Korean military – an apparent response to the arrival of an American nuclear-armed submarine in South Korea on Tuesday, the first such visit since 1981.