MIAMI (AFP) - US President Donald Trump on Friday (June 16) urged the nation to pray for Otto Warmbier, the university student detained in North Korea last year who was flown home this week in a coma.

"I'm glad Secretary of State Rex Tillerson and I, along with a very talented team, were able to get Otto Warmbier back with his parents. What's happened to him is a truly terrible thing," Trump said in Miami at the start of a speech on Cuba.

"But at least the ones who love him so much can now take care of him and be with him," he said, urging Americans to keep the family "in our hearts and prayers."

Warmbier, 22, suffered extensive tissue loss in all regions of his brain, doctors said Thursday, while adding they could not determine the cause of his injury.

He was incarcerated in March 2016 for stealing a political poster from a hotel.

The injury apparently occurred shortly thereafter.

Neurologist Daniel Kanter told a news conference in Warmbier's home city of Cincinnati, Ohio on Thursday that his neurological condition was best described "as a state of unresponsive wakefulness."

"He has spontaneous eye opening and blinking. However, he shows no signs of understanding language, responding to verbal commands or awareness of his surroundings," he said.

The doctors said Warmbier's severe brain injury was most likely - given his young age - to have been caused by cardiopulmonary arrest cutting the blood supply to the brain.

The university student, who had been on a tourist trip, was sentenced to 15 years hard labour, a punishment the US decried as far out of proportion to his alleged crime, accusing the North of using him as a political pawn.