By Antoinette Radford & Simon FraserBBC News
AFPSouth Korean soldiers stand guard in the village of Panmunjom in the Joint Security AreaNorth Korea is believed to have detained a serving US army soldier who crossed the heavily fortified border from South Korea without permission.
The man, identified by the Pentagon as Private 2nd Class (PV2) Travis King, 23, 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.
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 PV2 King had acted willingly by "making a run" across the border without authorisation, and the incident was being investigated by US Forces Korea.
On its travel advisory, the US tells its citizens not to travel to North Korea due to "the continuing serious risk of arrest" and the "critical threat of wrongful detention".
Hours after his detention, North Korea launched two suspected ballistic missiles into the nearby sea, however there has been no suggestion that it is tied to the soldier's detention.
South Korea's military confirmed the missile launch, which comes as tensions run high on the Korean peninsula.
It is unclear if the soldier has defected to North Korea or hopes to return, and there has been no word yet from the North.
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.
But he got into trouble there - officials in Seoul have confirmed that he spent two months in prison for assault charges.
The Yonhap news agency quoted "legal sources" as saying that he was fined for "repeatedly kicking" the back door of a police patrol vehicle in the capital's Mapo district, and shouted "foul language" at the police who apprehended him.
He was also suspected of punching a Korean national at a nightclub in September, the report said.
It is unclear if these were the reasons for his imprisonment.
PV2 King was released from prison on 10 July and was escorted to the airport for a US-bound flight.
Seoul officials said he passed through airport security but then somehow managed to leave the terminal and get on a tour of the border, from where he crossed over into North Korea.
Watch: Pentagon concerned about welfare of captured soldierAn eyewitness on the same tour told the BBC's US partner CBS News that 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 [North Korean] 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 authorities in the North 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 US student Otto Warmbier, 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.
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