Image copyright Reuters Image caption Otto Warmbier's parents say they went 15 months without hearing from or about their son

A team of US doctors have found "no sign of botulism" in the American student freed by North Korea after more than 15 months in captivity.

The regime had said the 22-year-old Otto Warmbier's coma was caused last year by botulism and a sleeping pill he took after his trial.

He has not spoken since his return to his family hometown in Ohio.

"His neurological condition can be best described as a state of unresponsive wakefulness," said Dr Daniel Kanter.

Mr Warmbier "shows no understanding of language" and has "extensive loss of brain tissue" which was likely caused by cardiopulmonary arrest, he said.

Image copyright EPA Image caption The doctors said they believe Mr Warmbier's condition was caused by respiratory arrest

According to scans taken after he arrived at the Cincinnati Medical Center earlier this week, there is no sign that he was physically abused during his detention, his doctors say.

Mr Warmbier's condition is "not what we normally see with traumatic brain injury. It's the type we see with cardiopulmonary arrest," Dr Kanter told reporters.

They believe respiratory arrest led to his condition, which is caused by a lack of oxygen and blood in the brain.

Earlier on Thursday, Mr Warmbier's father expressed doubts about the North Korean account of what caused the University of Virginia student's coma.

Media playback is unsupported on your device Media caption Otto Warmbier's dad: N Korea's treatment of my son 'horrible'

"Even if you believe their explanation of botulism and a sleeping pill causing the coma - and we don't - there's no excuse for a civilised nation to have kept his condition secret and to have denied him top notch medical care."

Otto Warmbier, an economics student from the University of Virginia, was arrested in January 2016 while visiting North Korea as a tourist.

He was given a 15-year prison sentence for attempting to steal a propaganda sign from a hotel.

What is botulism?

Botulism is a type of poisoning which can cause total body paralysis, difficulty in breathing and death in some cases.

It's caused by the Clostridium botulinum bacteria, but the symptoms of botulism are not from the bacteria themselves.

Instead, the microscopic organisms produce a powerful toxin which attacks the nervous system and causes paralysis.

That poison is called botulinum - which you may know for its commercial use in Botox, which removes wrinkles by paralysing facial muscles.

Botulism is contracted in two ways in adults - by eating food contaminated with the toxin, or through wounds.

The foodborne method happens when the bacteria are tinned or stored in food in another way that deprives them from air (which is when they produce the poison).

This rarely happens in modern Western food production, but is what North Korea claims happened to Mr Warmbier.