North Korea remains silent about the detention of an American soldier who sprinted across its heavily fortified border with South Korea, asother members of his tour group looked on in shock.

Observers said the North is unlikely to repatriate the soldier anytime soon amid heightened tensions between the rivals.

Private 2nd Class Travis King bolted into North Korea while on a tour of the Demilitarised Zone on Tuesday, a day after he was supposed to leave the country and travel to a base in the US.

Pte King was released from a South Korean prison earlier this month, not the first time he had run into legal trouble in the country. He could have faced more punishment in the US.

Pte King is the first known American held in North Korea in nearly five years. Each detention has set off complicated diplomatic wrangling, and this one comes at a time of heightened animosity.

On Wednesday, North Korea test-fired two ballistic missiles into the sea in an apparent protest at the deployment of a US nuclear-armed submarine in South Korea for the first time in decades.

“It’s likely that North Korea will use the soldier for propaganda purposes in the short-term and then as a bargaining chip in the mid to long-term,” Yang Moo-jin, president of the University of North Korean Studies in South Korea, said.

Pte King, 23, was a cavalry scout with the 1st Armoured Division who had served nearly two months in a South Korean prison for assault. He was released on July 10 and was supposed to leave on Monday for Fort Bliss, Texas, where he could have faced additional military discipline and discharge from the service.

He was escorted as far as customs but left the airport before boarding his plane.

It was not clear how he spent the hours until joining the Panmunjom tour and running across the border on Tuesday afternoon.

The US army released his name and limited information after Pte King’s family was notified.

One woman who was on the tour with Pte King said she initially thought his dash was some kind of stunt — and that she and others in the group could not believe what had happened.

Pte King’s stint in prison was not the first time he faced legal trouble in South Korea.

In February, a court fined him five million won (£3,060) after being convicted of assaulting an unidentified person and damaging a police vehicle in Seoul last October, according to a transcript of the verdict obtained by The Associated Press.

The ruling said Pte King had also been accused of punching a 23-year-old man at a Seoul nightclub, although the court dismissed that charge because the victim did not want the soldier to be punished.

Pte King’s mother told ABC News she was shocked when she heard her son had crossed into North Korea.

“I can’t see Travis doing anything like that,” Claudine Gates, of Racine, Wisconsin, said.

Ms Gates said said she had last heard from her son “a few days ago”, when he told her he would return soon to Fort Bliss.

She added that she just wanted “him to come home”.

White House press secretary Karine Jean-Pierre said that the US government was working with its North Korean counterparts to “resolve this incident”.

The American-led UN Command said on Tuesday that the US soldier was believed to be in North Korean custody.

“We’re closely monitoring and investigating the situation,” US defence secretary Lloyd Austin told a Pentagon news conference, noting that his foremost concern was for the soldier’s well-being.

Cases of Americans or South Koreans defecting to North Korea are rare, although more than 30,000 North Koreans have fled to South Korea to avoid political oppression and economic difficulties since a truce ended the 1950-53 Korean War.