When a member of a tour group visiting the Demilitarized Zone on South Korea's border suddenly sprinted away from the pack, a US soldier shouted: "Get him". But it was too late.

Private Second Class Travis King had crossed the border into nuclear-armed North Korea, where he is now believed to be in Pyongyang's custody, with the United States saying it was "very concerned" about how he would be treated.

New Zealand lawyer Sarah Leslie was on the same tour as King, and saw him break away from her group of about 40 to bolt across the divide.

"By the time I saw him he was running really fast and then, you know, not very long after that one of the American soldiers said 'Get him!' and then the rest of the Americans and the South Koreans all ran after him," she told AFP.

"But they didn't catch (him), and he was really close to the border by then, and he ran between two of those buildings that sit on the border," at which point the Americans and South Koreans had to abandon their pursuit, she said.

"I am not sure I've ever seen someone run that fast. He was running very fast," she said, adding that King had not seemed odd or given any inkling of his plan beforehand, that she had noticed.

- No North Koreans in sight -

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.

Leslie said she "didn't see any North Koreans the whole time (she) was there".

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

Leslie said the group had all been standing at the heart of Panmunjom, the truce village where then-president Donald Trump met North Korean leader Kim Jong Un in 2019 during a brief bout of diplomacy.

King had been a part of the tour group all day, but "was by himself, and didn't seem to be talking to anyone else", she said.

"The rest of us were just taking photos and talking. We were kind of waiting for the soldiers to tell us what to do next," Leslie said.

"I was just kind of looking at the North Korean side and someone ran from behind me, passed into my line of sight running really really fast," she said.

"I didn't notice it was him until he had already gone past me," she said.

- 'I thought he was crazy' -

Pyongyang has not commented on King's case nor responded to efforts by the United States -- which has no official diplomatic ties with Kim's regime -- to reach out, officials say.

North and South Korea remain technically at war since their 1950 to 1953 conflict ended with an armistice rather than a peace treaty, and the country's shared border is heavily fortified, including with minefields.

But at Panmunjom, the border is marked only by a low concrete divider.

"When I saw he kept going I thought he was crazy. I had no idea at the time he was a soldier. I could not think of any reason why anyone would do that," she said.

"On the bus out of the joint security area, that was when people were really looking at each other in shock," she said.

After King, who is reportedly 23 years old, disappeared into North Korean territory, the American soldiers told the other tourists to "run back into the building on the South Korean side", Leslie said.

"Everybody looked quite surprised... I guess nobody there, nobody expected that to happen. And we had been there quite a while when it happened," she said.

She said the soldiers escorting them were clearly surprised by the incident but "were very professional".

"They recovered their composure very quickly and they got us on the bus," she said, and went around explaining to people exactly what had happened, and trying to calm people down.

They all gave statements to the US military, and then headed back to Seoul.