The parents of US student Otto Warmbier have described North Korea as 'terrorists' who had 'systematically tortured' their son while he was detained by the pariah state.

Otto Warmbier, 22, was arrested in Pyongyang in 2016 after stealing a hotel sign, and detained in the country for 17 months. In June this year he was released on medical grounds but returned home to Cincinnati, Ohio in a coma, and died only days later.

North Korea has previously described Otto's death as a 'mystery'. It has denied allegations of torture and said that Warmbier's illness was a result of contracting botulism, a form of bacterial poisoning.

Otto's parents Fred and Cindy Warmbier said it was 'time to tell the truth about the condition that Otto was in' in an interview with Fox and Friends.

'Otto was systematically tortured and intentionally injured by Kim and his regime. This was no accident,' Mr Warmbier said.

He added: 'Otto had a shaved head, he had a feeding tube coming out of his nose, he was staring blankly into space, jerking violently. He was blind. He was deaf. As we looked at him and tried to comfort him it looked like someone had taken a pair of pliers and rearranged his bottom teeth.'

Otto was described as making 'inhuman' howling sounds, having arms and legs that were 'totally deformed' and a large scar on his foot.

He added: 'North Korea is not a victim, they are terrorists. They purposefully and intentionally injured Otto.'

US doctors has previously said there was no evidence of the botulism that North Korea had alluded to. But the case remains mysterious, as Otto's parents elected not to undertake a post-mortem examination, while the local news outlet The Cincinnati Enquirer said it had obtained a coroner's report on Warmbier which suggested no damage that would amount to evidence of torture, as BBC News reported. Otto's body was described as 'well-developed, well-nourished' and his teeth were reportedly in good condition.

President Trump tweeted following the interview, which was the first the parents had given since their son's death. He wrote: 'Great interview on @foxandfriends with the parents of Otto Warmbier: 1994 - 2017. Otto was tortured beyond belief by North Korea.'

Trump's comments have come at a time of increasingly heightened tensions between the US and North Korea, with both making threats of nuclear annihilation while the international community has urged dialogue and restraint.