Update 9pm: Doctors who are treating an American college student released by North Korea in a coma say he has severe injury to all regions of his brain. They described 22-year-old Otto Warmbier as in a state of "unresponsiveness wakefulness".

They say he does not show any consistent response to stimulation, shows no sign of understanding language, responding to commands or awareness of his environment.

Doctors with the University of Cincinnati Health system say Mr Warmbier shows extensive loss of brain tissue, consistent with respiratory arrest, when the brain is cut off from oxygen but they are not sure why. They said his prognosis remains confidential.

Earlier: An American college student who was released by North Korea in a coma has suffered a "severe neurological injury", a spokeswoman for the Ohio hospital treating him said.

Otto Warmbier is in stable condition after arriving at the University of Cincinnati Medical Centre two days ago. Doctors plan to hold a news conference on campus later.

The 22-year-old student's father, Fred Warmbier, said he does not believe North Korea's explanation that his son's coma resulted from botulism and a sleeping pill.

Fred Warmbier said there is relief that his son is home, but also anger that he was so brutally treated for so long.

Mr Warmbier said his son was "terrorised and brutalised" during his 17-month detention, and has been in a coma for more than a year.

He said he and his wife, Cindy, only learned of their son's condition last week.

The University of Virginia student was medically evacuated from North Korea and arrived in Cincinnati late on Tuesday. He was then taken by ambulance to a hospital.

Mr Warmbier was serving a 15-year prison term with hard labour in North Korea after he tearfully confessed that he tried to steal a propaganda banner while visiting the country.

Such detentions in the totalitarian nation have added to tensions between Washington and Pyongyang. Three Americans remain in custody.

The US government accused North Korea of using such detainees as political pawns.

North Korea accuses Washington and South Korea of sending spies to overthrow its government.

US secretary of state Rex Tillerson said his department is continuing "to have discussions" with North Korea about the release of the other three imprisoned American citizens.

- PA