A United States national entered North Korea during a tour of the heavily fortified border and is believed to have been detained, the United Nations Command said Tuesday.
"A US national on a JSA orientation tour crossed, without authorisation, the Military Demarcation Line into the Democratic People's Republic of Korea (DPRK)," the UN Command said, referring to the Joint Security Area and the border between the Koreas.
"We believe he is currently in DPRK custody and are working with our KPA counterparts to resolve this incident," it said, referring to the North's Korean People's Army.
North and South Korea remain technically at war since 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 destination and hundreds of visitors tour the area on the South Korean side every 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 by stepping across the demarcation line there.
"Panmonjom is the most likely site this American chose to cross into North Korea because it's the only location one could attempt such a move out of the whole JSA tour," Choi Gi-il, a professor of military studies at Sangji University, told AFP.
South Korea's SBS television station reported that the person who crossed the border was a US soldier. South Korea's defence ministry declined to comment when contacted by AFP.
An eyewitness who said they were on the same 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 NK soldiers -
North Korea sealed its borders at the start of the Covid-19 pandemic in 2020 and has yet to reopen. 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 run across the border to retrieve the US national.
Retired US Army Lieutenant Colonel Steve Tharp, who worked in the JSA area, told Seoul-based specialist site NK News he had no idea how the North Koreans would react to the incident because there was "so little data out there" about events such as this.
"This is the first contact since Covid... We don't know what they're thinking," he told NK News.
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 weapons.
Seoul and Washington have ramped up defence cooperation in response, staging joint military exercises with advanced stealth jets and US strategic assets.
- Nuclear sub -
The allies held the first Nuclear Consultative Group meeting in Seoul on Tuesday and announced an American nuclear submarine was making a port visit to Busan for the first time since 1981.
The move is likely to trigger a strong response from North Korea, which baulks at having US nuclear assets deployed around the Korean peninsula.
Kim Yo Jong, the North Korean leader's powerful sister, said Monday such actions would only "make the DPRK go farther away" from possible talks.
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 as they sought to prevent his escape but he survived after hours of surgery.
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.