Family says Otto Warmbier has been in a coma since March 2016, shortly after being sentenced to 15 years of hard labour.

An American university student who was returned to the US this week after being held in North Korea for 18 months, has a severe brain injury and is in a state of “unresponsive wakefulness”, doctors say.

Otto Warmbier, 22, has been in a coma since March 2016, shortly after he was sentenced to 15 years of hard labour in North Korea, according to his family.

He arrived in the US on Tuesday and is stable but “shows no sign of understanding language, responding to verbal commands or awareness of his surroundings”, said Dr Daniel Kanter, medical director of the neuroscience intensive care unit at the University of Cincinnati Medical Center, on Thursday.

“He has not spoken,” Kanter said.

He said Warmbier was breathing on his own, but “he has not engaged in any purposeful movements or behaviours. He has spontaneous eye opening and blinking.”

Warmbier, from Wyoming, Ohio, was arrested for trying to steal an item bearing a propaganda slogan, North Korean media reported.

He was visiting North Korea with a tour group.

At a news conference before his trial, a weeping Warmbier said he had made “the worst mistake of my life” and pleaded to be released.

On Thursday, North Korea said that it had released Warmbier “on humanitarian grounds”.

His father, Fred Warmbier, said the family was proud of him, calling him “a fighter”.

Warmbier’s parents were told their son was given a sleeping pill soon after his trial in March last year but never woke up.

Botulism and sleeping pill

A Washington Post newspaper report said the parents were told he may have been infected by botulism while in the North Korea jail system.

The elder Warmbier said he did not believe North Korea’s explanation that the coma resulted from botulism and 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.

Kanter said Otto Warmbier suffered “extensive loss of brain tissue in all regions of the brain”.

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Doctors said his injuries are consistent with respiratory arrest cutting off oxygen to the brain, but they do not know what caused it.

The developments come amid tensions with the US following a series of missile tests by North Korea, focusing attention on an arms build-up that Pentagon chief Jim Mattis this week described as “a clear and present danger to all”.

A state department spokeswoman said in Washington, DC that Warmbier’s release followed “quiet diplomacy”, at US President Donald Trump’s suggestion.