The father of Otto Warmbier, the US student who was detained in North Korea for 15 months, has cast doubt on the regime's explanation for his son's coma, saying they treated him "brutally."

North Korea said the 22-year-old's medical condition was caused by botulism and a sleeping pill he was given after his trial in 2016.

Otto Warmbier is currently in an Ohio hospital suffering from "severe neurological injury" but is in a stable condition after North Korea released him on "humanitarian grounds".

His father Fred Warmbier said: "We went for 15 months without a word from or about Otto.

"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.

"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."

What is botulism?

Accoring to the NHS, botulism is a very rare but life-threatening condition caused by toxins produced by Clostridium botulinum bacteria.

These toxins are some of the most powerful known to science. They attack the nervous system (nerves, brain and spinal cord) and cause paralysis.

Most people will make a full recovery with treatment, but the paralysis can spread to the muscles that control breathing if it's not treated quickly.

Warmbier, an economics student from the University of Virginia, was given a 15-year prison sentence after attempting to steal a propaganda sign from a hotel.

He was freed hours after US basketball star and "friend" of Kim Jong-un Dennis Rodman arrived in the country.

Three other US nationals are still being held by the Kim Jong-un administration. Two of them were teachers at a private school while the third person worked in a special economic zone in North Korea.

It is believed that North Korea has been using the detentions as a bargaining chip in its negotiations with Washington. The US wants North Korean dictator Kim Jong-un to abandon his nuclear and missile programmes.