The man was on an organised tour of the UN-run zone dividing the two countries.
The crisis comes during a particularly tense time with the North, one of the world’s most isolated states. The US tells its citizens not to go there.
A senior US commander said there had been no contact with the soldier.
Admiral John Aquilino Commander of the US Indo-Pacific Command said he was “not tracking” contact with North Korea. He said the soldier had acted willingly by “making a run” but without authorisation, and the incident was being investigated by US Forces Korea.
Hours after the soldier’s detention, North Korea launched two suspected ballistic missiles into the nearby sea.
The missile launch, which has been confirmed by South Korea’s military, comes as tensions run high on the Korean peninsula. There has been no suggestion that the launch is tied to the soldier’s detention.
It is unclear if the man has defected to North Korea or hopes to return. There has been no word yet from the North.
The Pentagon has identified the soldier as Private 2nd Class (PV2) Travis King. In a statement, a Pentagon spokesperson said that PV2 King had been in the army since January 2021.
He is a cavalry scout – a reconnaissance specialist – originally assigned to an element of the army’s 1st Armoured Division on a rotation with the US military in South Korea.
The Associated Press reports that PV2 King was apparently facing disciplinary action after being held in South Korea on assault charges.
According to the BBC’s US partner CBS News, PV2 King passed through airport security in Seoul but somehow managed to leave the terminal and get on a tour of the border, from where he crossed over.
The American military has said he did so “wilfully and without authorisation”.
Watch: Pentagon concerned about welfare of captured soldier
An eyewitness on the same tour told CBS they had visited a building at the border site – reported by local media to be the truce village of Panmunjom – 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.
The United Nations Command, which operates the Demilitarised Zone and joint security area (JSA), said earlier its team had made contact with the North Korean military to try to negotiate his release.
“We believe he is currently in DPRK custody and are working with our KPA [Korean People’s Army – North Korea’s military] counterparts to resolve this incident,” it said.
It is unclear where or in what conditions PV2 King is being held.
Greg Scarlatoiu, executive director of the Washington DC-based Committee for Human Rights in North Korea, told the BBC that North Korean authorities were likely to “try pump information out of him” about his military service and “try to coerce him into becoming a propaganda tool”.
The Demilitarised Zone (DMZ) separates the two Koreas and is one of the most heavily fortified areas in the world.
It is filled with landmines, surrounded by electric and barbed wire fencing and surveillance cameras. Armed guards are supposed to be on alert 24 hours a day.
The DMZ has separated the two countries since the Korean War in the 1950s, in which the US backed the South. The war ended with an armistice, meaning that the two sides are still technically at war.
Dozens of people try to escape North Korea every year, fleeing poverty and famine, but defections across the DMZ are extremely dangerous and rare. The country sealed its borders in 2020 at the start of the Covid-19 pandemic and has yet to reopen them.
The last time a soldier defected at the JSA was in 2017, when a North Korean soldier drove a vehicle, then ran by foot across the military demarcation line, South Korea said at the time. The soldier was shot at 40 times, but survived.
Before the pandemic more than 1,000 people fled from North Korea to China every year, according to numbers released by the South Korean government.
The detention of the soldier presents a major foreign policy headache for US President Joe Biden. PV2 King is believed to be the only American citizen currently in North Korean custody. Six South Koreans remain in detention there.
Relations between the US and the North plummeted in 2017 after a US student who had been arrested a year earlier for stealing a propaganda sign was returned to the US in a comatose state and later died. His family blames the North Korean authorities for his death.
Three US citizens were later freed during Donald Trump’s presidency in 2018. But ultimately, a series of talks held between Kim Jong Un and the former US president did little to improve the relationship.
North Korea has since tested dozens of increasingly powerful missiles that could carry nuclear warheads, which have been met by a slew of sanctions by the US and its allies.
The detention of the US national comes on the same day as a US nuclear-capable submarine docked in South Korea for the first time since 1981.
The submarine was specifically supplied to help the country deal with the nuclear threat posed by North Korea. Ahead of its deployment there were threats of retaliation from the authorities in Pyongyang, which warned the US that sending nuclear weapons to the peninsula could spark a nuclear crisis.
Hours after PV2 King’s detention, South Korea’s military confirmed that two ballistic missiles were launched from North Korea and landed outside Japan’s exclusive economic zone.
