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

The emotional father of a US student freed by North Korea this week says he does not believe the regime's explanation for his son's coma.

The regime said 22-year-old Otto Warmbier's medical condition was caused last year by botulism and a sleeping pill he was given after his trial.

But Fred Warmbier said his son had been treated "brutally".

He spoke as an Ohio hospital said Otto had suffered a "severe neurological injury" and is in a stable condition.

"We went for 15 months without a word from or about Otto," Mr Warmbier said of his son, who fought back tears at Thursday's press conference in Cincinnati, Ohio.

"It was only a week ago that the North Korean government now claims that he was in a coma for almost all of that time.

Image copyright Reuters Image caption Medical personnel were seen transferring a person believed to be Mr Warmbier to an ambulance at an airport in Cincinnati on Tuesday night

"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.

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.

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

Fred Warmbier told reporters his son had been held as a "war criminal'.

He said he and his wife, Cindy, had relied "on this false premise that they would treat Otto fairly and let him go".

Image copyright Getty Images Image caption Fred Warmbier says he never heard about or from his son during his 15-month detention

In honour of his son, Fred Warmbier wore the same jacket Otto was wearing when he was paraded before media to tearfully confess to attempting to take the sign as a "trophy" for a US church.

Mr Warmbier also appeared to criticise the Obama administration for failing to help secure his son's release.

How harsh is prison in North Korea?

"When Otto was first taken we were advised by the past administration to take a low profile while they worked to obtain his release," he said.

"We did so without result. Earlier this year Cindy and I decided the time for strategic patience was over."

The university student was freed hours after US basketball star Dennis Rodman arrived in North Korea, but Fred Warmbier said the professional athlete's visit had nothing to do with his son's release.

Media playback is unsupported on your device Media caption US student Otto Warmbier said he was "a poor scapegoat" before he was jailed in 2016

The US has in the past accused North Korea of detaining Americans as political pawns in negotiations over its nuclear weapons programme.

Pyongyang has accused Washington and South Korea of sending spies to overthrow its regime.

Three Americans remain in custody in North Korea.

The detentions have come at a time of heightened tension between North Korea and the US and its regional neighbours.