A US soldier who served around two months in a South Korean jail on assault charges was believed to be in North Korean custody Wednesday after crossing the heavily fortified border without authorisation, officials said.

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 authorisation," 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.

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 United States 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 Demilitarised Zone running along the border.

Soldiers from both sides face off at the JSA north of Seoul, which is overseen by the United Nations Command. It is also a popular tourist site, and hundreds of visitors tour the South Korean side each day.

Former US president Donald Trump met North Korean leader Kim Jong Un at the Panmunjom Truce Village in 2019 and even stood on North Korean soil after stepping across the demarcation line there.

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 realised it wasn't a joke and then everybody reacted and things got crazy," they said.

- No North Korean soldiers -

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.

The ballistic missile launches were likely unrelated to the American soldier crossing the border, "but such an incident doesn't help matters," said Leif-Eric Easley, a professor at Ewha University in Seoul.

"The Kim regime is likely to treat a border crosser as a military, intelligence and public health threat even though it is more likely that such an individual is mentally distressed and acting impulsively due to personal issues."

North Korea sealed its borders at the start of the Covid-19 pandemic in 2020 and has yet to reopen them. Its security presence on its side of the border at the JSA has also been scaled back significantly.

When AFP toured the JSA earlier this year, no North Korean guards were visible in the area. Even so, under armistice protocols, South Korean or US personnel could not cross the border to retrieve the US national.

The incident comes as relations between the two Koreas are at one of their lowest points, with diplomacy stalled and Kim calling for increased weapons development, including tactical nuclear warheads.

Seoul and Washington have ramped up defence cooperation in response, staging joint military exercises with advanced stealth jets and US strategic assets.

- JSA -

The JSA in Panmunjom is typically peaceful despite ongoing hostility between the two sides.

In 1976, two American soldiers were killed in the JSA by North Koreans with axes in a dispute over a tree.

The last time there was a defection at the JSA was in 2017, when a North Korean soldier drove a military jeep and then ran on foot across the demarcation line at Panmunjom.

He was shot multiple times by his fellow North Korean soldiers but survived.

In general, defections between the two Koreas are rare but far more common in the other direction, when North Koreans seek to escape grinding poverty and repression by fleeing, typically across the northern land border into China.