North Korea allowed a student from the United States who fell into a coma while imprisoned in a labour camp to be flown home on Tuesday, as Washington stepped up efforts to halt Pyongyang's nuclear programme.

The release of 22-year-old Otto Warmbier, 18 months into a 15-year sentence, came as US President Donald Trump invited South Korea's new leader Moon Jae-In to the White House for talks on the nuclear stand-off.

Secretary of State Rex Tillerson said his department had "secured" Wambier's release in talks with North Korea and is pushing for three more Americans to be freed. It was not immediately clear if he had made any concessions.

"Otto has left North Korea," his parents, Fred and Cindy Warmbier, said in a statement to CNN. "He is on medivac flight on his way home. Sadly, he is in a coma and we have been told he has been in that condition since March of 2016. We learned of this only one week ago."

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Warmbier's parents were told their son was given a sleeping pill soon after his trial in March last year but never woke up. A Washington Post report says the parents had been told he may have been infected by botulism while in the North Korea jail system.

Warmbier, a student at the University of Virginia, was arrested for trying to steal a propaganda banner from a wall in a North Korean hotel during a visit.

He was detained at the airport as he was leaving the country with a tour group in January 2016.

Bill Richardson, a veteran former diplomat and politician who has played a role in past negotiations with North Korea, welcomed Warmbier's release but said "we are deeply concerned regarding his health".

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Tillerson told US senators at the start of a budget hearing that the State Department had no comment on Mr Warmbier's condition, "out of respect for him and his family."

The US had accused the North of using Warmbier as a political pawn, and condemned the sentence as far out of proportion to his alleged crime.

The announcement came amid tension between Washington and Pyongyang following a series of missile tests by the North, focusing attention on an arms build-up that Pentagon chief Jim Mattis on Monday dubbed "a clear and present danger to all".

Almost immediately on taking office in January, Trump and his team - having been briefed by outgoing leader Barack Obama - declared the North's attempts to build, test and arm a nuclear-capable ballistic missile to be Washington's biggest threat.

Washington has stepped up pressure on China and other foreign powers to enforce existing UN sanctions, and has deployed increased military assets of its own in the region.

'Worst mistake of my life'

At a news conference before his trial, a sobbing Warmbier said he had made "the worst mistake of my life" and pleaded to be released.

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The other US citizens currently being held by North Korea include Kim Hak-Song and Tony Kim, both professors at the Pyongyang University of Science and Technology.

They were arrested this year for "hostile acts" and trying to "overturn" the government.

Korean-American Kim Dong-Chul was arrested in 2015 and sentenced to 10 years' hard labour on charges of subversion and espionage.

The North has occasionally jailed US citizens and released them only after visits by high-profile political figures, including former president Bill Clinton.

Meanwhile, the White House said Moon and Trump will meet on June 29 and 30 to discuss ways of building on what America frequently calls its "ironclad" alliance with South Korea.

Moon, a centre-left politician who was sworn in last month after a landslide election win, wants to engage with the North to bring it to the negotiating table, rather than continuing the hardline stance taken by his ousted predecessor Park Geun-hye.

Dennis Rodman arrives in Pyongyang 

The news of Warmbier's release came as the flamboyant retired NBA basketball star Dennis Rodman - a former contestant on Trump's Celebrity Apprentice reality show - flew to Pyongyang to resume his quixotic quest to broker detente between the US and North Korea

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The star has visited the reclusive country at least four times before, most recently in 2014 when he attracted a deluge of criticism after being filmed singing happy birthday to his "friend for life", leader Kim.

Before arriving this time, Rodman told reporters that Trump would be happy with the trip, since he was "trying to accomplish something that we both need", sparking speculation that he may be operating as an unofficial envoy.

US officials dismissed this, saying he was travelling as a private citizen, but the basketball icon is probably Kim and Trump's only mutual friend.

Source: News agencies