Private Second Class Travis T. King was reportedly facing disciplinary action by the US military when he crossed into the secretive country, US officials said.

His mother Claudine Gates of Racine, Wisconsin, said she was shocked when she heard her son was in North Korea.

"I can't see Travis doing anything like that," she told ABC News

Ms Gates said that the Army told her on Tuesday morning that her son had crossed into the North, adding that she last heard from her son "a few days ago," when he told her he would return soon to his base in Fort Bliss.

She went on to say that she just wants "him to come home".

The crossing has created a fresh crisis for Washington in its dealings with the nuclear-armed state.

King crossed after joining an orientation tour of the Joint Security Area, which is part of the 160-mile demilitarised zone separating South and North Korea.

The US Army said the soldier "wilfully and without authorisation crossed the Military Demarcation Line into the Democratic People's Republic of Korea (DPRK)".

The man was with a group of visitors, including civilians, to the Panmunjom truce village when he suddenly bolted over the brick line marking the border, South Korean media reported, citing South Korean army sources.

Two US officials, who spoke on condition of anonymity, said the soldier had been due to face disciplinary action by the US military.

However, the officials also told Reuters it was not clear why the soldier fled to North Korea.

He had finished serving time in detention in South Korea for breaking an unspecified rule and was returning to his home unit in the US, two officials said.

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The soldier was not in US military custody at the time he decided to flee, one official said, and it appeared he made a surprise decision to join the civilian tour to the demilitarised zone.

It was unclear for how long North Korean authorities would hold the soldier but analysts said the incident could be valuable propaganda for the isolated country.

The US bans its citizens from entering North Korea - the notorious totalitarian state run by Kim Jong Un where millions live in hunger and poverty.

"Historically, the North holds these folks for weeks, if not months, for propaganda purposes, especially if this is a US soldier, before a coerced confession and apology," said Victor Cha, a former US official and Korea expert at the Center for Strategic and International Studies.

"It also sometimes requires an American official or ex-official to travel there to obtain the release," he added.

US Defense Secretary Lloyd Austin told a news briefing in an update: "There's a lot that we're still trying to learn.

"We believe that he is in (North Korean) custody and so we're closely monitoring and investigating the situation and working to notify the soldier's next of kin."

Mr Austin has also said his "foremost concern" is the soldier's welfare.

Meanwhile, White House spokesperson Karine Jean-Pierre said US officials at the Pentagon, the State Department and the United Nations were all working to "to ascertain more information and resolve this situation."

"We're in the early stages," she said, adding that the primary concern was determining the wellbeing of the soldier.

South and North Korean forces face each other at the Joint Security Area and it's often used for diplomatic meetings between the two countries.

It is also a popular tourist spot.

The soldier's crossing comes at a time of high tensions on the Korean peninsula, with the arrival of a US nuclear-armed ballistic missile submarine in South Korea for a rare visit.

The move is seen as a warning to North Korea over its own military activities. It has been testing increasingly powerful missiles capable of carrying nuclear warheads, including a new solid-fuel intercontinental ballistic missile launched last week.

It fired another ballistic missile into the sea near Japan on Tuesday, South Korea's Yonhap news agency reported, citing South Korea's military.