WASHINGTON/CINCINNATI • A flurry of secret diplomatic contacts led to the release of American college student Otto Warmbier who spent more than 17 months in detention in North Korea.

The release of Mr Warmbier, 18 months into a 15-year sentence, came as US President Donald Trump invited South Korea's new leader Moon Jae In to Washington for talks later this month on the escalating stand-off over the North's nuclear programme.

Secretary of State Rex Tillerson on Tuesday said his agency had "secured" the 22-year-old's release in talks with North Korea and is pushing for three more Americans to be freed. It was not clear if the US had made any concessions.

The three Americans still being held were in fairly healthy condition and were allowed to meet the State Department's top official on North Korea, Mr Joseph Yun, when he travelled to Pyongyang earlier this week.

Mr Warmbier's imprisonment had come up in May as part of continuing negotiations in Oslo during the Track 2 talks - unofficial discussions primarily involving former senior officials from both sides - about how the United States and North Korea could start resolving their differences, particularly over Pyongyang's nuclear and missile programmes.

The subject of allowing Swedish officials to visit US prisoners in North Korea was discussed at length. A short time later, a Swedish representative was allowed to see one of three other Americans held in North Korea. It was after that visit that Pyongyang urgently requested the meeting with US officials in New York.

On June 6, Mr Yun met the North's ambassador to the United Nations and was told about Mr Warmbier's condition, said an American official. Mr Tillerson consulted with President Trump and arrangements were made for Mr Yun and a medical team to travel to Pyongyang, the official said.

Mr Yun arrived on Monday, visited Mr Warmbier with two doctors and demanded his release on humanitarian grounds. The North Koreans agreed and the college student was flown out on Tuesday.

A military airplane carrying Mr Warmbier landed in his hometown Cincinnati, Ohio, late on Tuesday, and he was rushed to the University of Cincinnati Medical Centre for urgent treatment.

He is in "bad shape",a source close to the family told CNN.

The student has been in a coma for "over a year now and urgently needs proper medical care", said Mr Bill Richardson, the former governor of New Mexico, who has been in touch with the Warmbier family and has served as a negotiator with the North Korean government.

Mr Warmbier's family said they were told by North Korean officials, through contacts with American envoys, that he fell ill from botulism some time after his March 2016 trial and lapsed into a coma after taking a sleeping pill.

The New York Times quoted a senior US official as saying Washington recently received intelligence reports that he had been repeatedly beaten in custody.

Mr Warmbier, a student at the University of Virginia, was detained in January last year and sentenced two months later to 15 years of hard labour for trying to steal an item with a propaganda slogan, according to North Korean media.

News of Mr Warmbier's release surfaced after the flamboyant retired NBA basketball star Dennis Rodman - a former contestant on Mr Trump's "Celebrity Apprentice" reality show - flew to Pyongyang on Tuesday to resume his quixotic quest to broker detente between the US and Mr Kim Jong Un's authoritarian regime. He has visited the North at least four times before.

State Department spokesman Heather Nauert said Mr Rodman's visit "had nothing to do with the release" of Mr Warmbier.

REUTERS, NYTIMES, WASHINGTON POST, AGENCE FRANCE-PRESSE