The American college student who was released by North Korea in a coma blinks his eyes but "shows no signs of understanding language" and has lost tissue in all areas of his brain, doctors in Cincinnati said at a news conference on Thursday afternoon.

Otto Warmbier hasn't needed a respirator or supplemental oxygen since arriving at the University of Cincinnati Medical Center late Tuesday night by ambulance, but has a severe neurological injury, they said.

"He has spontaneous eye opening and blinking," said Dr. Daniel Kanter, director of neurocritical care for the University of Cincinnati Health system.

"However, he shows no signs of understanding language, responding to verbal commands or awareness of his surroundings. He has not spoken."

Daniel Kanter, medical director of the neuroscience intensive care unit, right, speaks alongside Jordan Bonomo, a neurointensivist, left, during a news conference regarding Otto Warmbier's condition Thursday at University of Cincinnati Medical Center in Cincinnati. (John Minchillo/Associated Press)

Doctors said they can't speculate on what happened to Warmbier in North Korea, but that he likely suffered a respiratory arrest at some point in time, which could be the result of several causes. They found no evidence of an "acute or healing fracture" after examining several parts of his body and skull.

'Remarkable courage' of family

Hospital officials refused to speculate on Warmbier's long-term prognosis, citing privacy with the patient's family, but praised the "remarkable courage" of the Warmbiers.

His father, Fred Warmbier, said Thursday he does not believe North Korea's explanation that his son's condition is the result of botulism and a sleeping pill.

Dr. Brandon Foreman of University of Cincinnati Health said tests conducted at the hospital did not show the active presence of botulism.

Doctors are dealing with a significant gap, as Warmbier was in captivity for 18 months. They were provided a scan from April 2016, which indicated the injury to the brain had already occurred.

Fred Warmbier said he's relieved his son is home with those who love him, but angry he was so brutally treated for so long.

Fred Warmbier, father of Otto Warmbier, speaks during a news conference Thursday at Cincinnati's Wyoming High School, where his son attended. (John Minchillo/Associated Press)

"Otto has been terrorized and brutalized for 18 months by a pariah regime in North Korea," the father said in an interview with Tucker Carlson of Fox News that aired Thursday night.

Warmbier was serving a 15-year prison term with hard labour in North Korea after tearfully confessing to trying to steal a propaganda banner while on a visit

"The day after he was sentenced, he went into a coma," Fred Warmbier said in the Fox interview. He said he and his wife, Cindy, only learned of their son's condition last week.

'Tragic situation'

Former New Mexico governor Bill Richardson, also a former U.S. ambassador to the United Nations, said there should be an investigation into what led to this "tragic situation."

Richardson, a Democrat, credited the Department of State with securing Warmbier's return from North Korea without any preconditions, but said a forceful response from the U.S. government would be required "if it's determined that there was a coverup, and Otto's condition was not disclosed and he didn't get proper treatment."

Such detentions in the totalitarian country have added to tensions between Washington and Pyongyang.

Three American citizens remain in custody:

Kim Dong Chul, a businessman.

Tony Kim, a university professor.

Kim Hak-song, a university employee.

The U.S. government accuses North Korea of using such detainees as political pawns. North Korea accuses Washington and South Korea of sending spies to overthrow its government.

Canadian Hyeon Soo Lim is shown in mid-2015 in Pyongyang, North Korea. Lim's current status is unknown. (Associated Press)

U.S. Secretary of State Rex Tillerson said Tuesday his department was continuing "to have discussions" with North Korea about the release of the other three imprisoned Americans.

U.S. President Donald Trump is aware of the situation. Fred Warmbier said he received "a very nice phone call" Wednesday from the president.

Hyeon Soo Lim, a pastor who has lived in the Toronto area for three decades after emigrating from South Korea, has been detained in North Korea since 2015. Before his detention, he had made dozens of trips to North Korea for humanitarian aid purposes, according to his church and family.