US student Otto Warmbier suffered extensive brain damage while detained in North Korea

An American college student who emerged from prison in North Korea in a coma has severe brain damage, but doctors don't know what caused it, a medical team treating him in Ohio has said.

The doctors described Otto Warmbier as being in a state of “unresponsive wakefulness” but declined to discuss his outlook for improvement, saying such information would be kept confidential.

“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.”

At an earlier press conference, Otto's father said does not believe the North Korea’s claims about his son’s health, but feels the “results speak for themselves” when asked if Barack Obama’s administration could have done more to bring his son home sooner.

Fred Warmbier, wearing the jacket his son wore to his March 2016 trial in North Korea, gave a news conference in Cincinnati, Ohio just a few days after his son arrived home in a coma.

North Korean authorities have said that Otto, 22, fell ill from botulism sometime after his the trial and fell into a coma after taking a sleeping pill. US doctors said they found no evidence of active botulism, a rare, serious illness caused by contaminated food or a dirty wound.

Mr Warmbier said his family did not believe the claims from the “pariah regime” and though they are thrilled to have their son home, there is “anger that he was so brutally treated for so long”.

Otto, a student at the University of Virginia, went with a tour group from Beijing to Pyongyang in January 2016 despite a long-standing US State Department travel warning and imprisonment of Americans in the past.

He was arrested and convicted while trying to leave the country after his group's “New Year’s Party Tour”.

Mr Warmbier said the tour group “provides fodder” for the regime.

He described his son as a “young, thrill-seeking, great kid” who was “lure[d]” by North Korea through groups like this.

Mr Warmbier said the regime leads people to believe that no Americans are ever imprisoned and that he and his wife “agreed to let” Otto go on the trip.

The charge against Otto, according to North Korean news agency, was “perpetrating a hostile act against” the Democratic People's Republic of Korea – the official name for the reclusive Communist nation.

He allegedly attempted to steal a propaganda banner from the hotel in which he was staying, but it remains unclear whether this is true.

Mr Warmbier said he does not think the family will ever find out the “real reason” his son was released, but he believes the US State Department was “negotiating pretty tough with them”.

He praised the efforts of Secretary of State Rex Tillerson and Ambassador Joseph Yun, the Special Representative for North Korea Policy.

He also said that his son’s release had nothing to do with former US basketball star Dennis Rodman’s trip to visit North Korean leader Kim Jong-un.“My family has been brutalised the last 18 months,” Mr Warmbier said.

He said that there was little in the way of updates on his son for much of that time and that they were “forced to be quiet and act different because we didn’t want to offend” North Korea should it impact negotiations for Otto’s release.

“Could the previous administration have done more? I think the results speak for themselves,” said Mr Warmbier, who also said he never had a face-to-face meeting with Mr Obama but did not confirm any conversations or meetings with then-Secretary of State John Kerry.

One of Obama's advisers, Ned Price, said the Obama administration had “no higher priority” than securing the release of Americans detained overseas but North Korea's isolation “posed unique challenges.” He said Obama “worked through every avenue available” to try to secure Otto's release.

Mr Warmbier said he had a “nice conversation” with Donald Trump.

The President asked how the family was doing and described how the administration “worked hard” for his son’s release. Mr Warmbier said Mr Trump was “kind...gracious”.

Defence Secretary James Mattis declared North Korea “most urgent and dangerous threat to peace and security,” before the House Armed Services Committee, just hours before Mr Warmbier's release was announced.

“North Korea’s continued pursuit of nuclear weapons and the means to deliver them has increased in pace and scope,” Mr Mattis said.

Mr Warmbier commented on the need for a "tough approach" to North Korea because they "have proven they are not nature's noble men".

The Ohio hospital where Otto was taken has said that he is in stable condition but suffered “severe neurological injury”.

Mr Warmbier said his family is “trying to make [Otto] comfortable”.