King ran across the border on July 18 after joining a sightseeing tour of the Demilitarised Zone between the two Koreas.

Last month, Pyongyang confirmed it was holding the US soldier, saying King had defected to North Korea to escape "mistreatment and racial discrimination in the U.S. Army".

But after completing its investigation, Pyongyang has "decided to expel Travis King, a soldier of the U.S. Army who illegally intruded into the territory of the DPRK, under the law of the Republic", the Korean Central News Agency said Wednesday, using the North's formal name.

It did not give any details about where or when King would be released.

After a drunken pub fight, an incident with police and a stay in South Korean jail, Private Second Class King was being taken to the airport in July to fly back to Texas.

But instead of travelling to Fort Bliss for disciplinary hearings, King snuck away, joined a Demilitarised Zone sightseeing trip and slipped over the border.

King's border crossing came with relations between the two Koreas at one of their lowest points ever, with diplomacy stalled and North Korean leader Kim Jong Un calling for increased weapons development, including tactical nuclear warheads.

Seoul and Washington have ramped up defence cooperation in response, staging joint military exercises with advanced stealth jets and US strategic assets.

- Rare defections -

The two Koreas remain technically at war because the 1950-53 conflict ended in an armistice, not a treaty, and most of the border between them is heavily fortified.

But the Joint Security Area where King made his escape, the frontier is marked only by a low concrete divider and is relatively easy to cross, despite the presence of soldiers on both sides.

Pyongyang has a long history of detaining Americans and using them as bargaining chips in bilateral negotiations.

One of the last US citizens to be detained by the North was student Otto Warmbier, who was held for a year-and-a-half before being released in a coma to the United States. He died six days later.

Around half a dozen American soldiers made rare defections to the North after the Korean War and were used for the country's propaganda.

In one such case, US soldier Charles Robert Jenkins crossed into the North in 1965, drunk after 10 beers, while patrolling the DMZ in an attempt to avoid facing combat duty in Vietnam.

Although he quickly regretted his defection, Jenkins was held for decades, teaching English to North Korean soldiers and appearing in propaganda leaflets and films.

He was eventually allowed to leave in 2004 and subsequently spoke out about the dire conditions of life in the North until he died in 2017.