Expert believes U.S. university student died from a VIRUS contracted in savage prison camps where dead bodies are used to fertilise food for starving inmates

A retired Australian judge who interviewed survivors of North Korean torture camps says Otto Warmbier was jailed in a prison where dead bodies had been turned into fertiliser.

The observation comes days after Mr Warmbier, a 22-year-old University of Virginia student, died from a brain injury after spending 18 months behind bars for stealing a communist propaganda poster.

Justice Michael Kirby, a retired Australian High Court judge, said the young man from Cincinnati, Ohio was likely to have caught a virus in prison where 'the conditions are extremely harsh'.

'It doesn't appear to have been actual physical violence to the brain but it does appear to be some virus or bacteria and whatever it was, it led to his loss of consciousness, coma, return to the United States and death,' he told Sky News Australia on Thursday.

Mr Warmbier died at the University of Cincinnati Medical Centre, six days after being flown in a private jet from North Korea, where he was sentenced to 15 years of hard labour for stealing a propaganda poster from a hotel lobby.

Justice Kirby said this 'ordinary prison' by North Korean standards was where the bodies of dead inmates had been turned into fertiliser during the hermit kingdom's famine of the 1990s.

'During the famine, everyday, people were put outside the cells and picked up, emaciated and put onto a wheelbarrow and wheeled to an area where a vat was set up and their bodies were put in the vat and reduced to ashes and the ashes and spare body parts were sprinkled on the fields next to the prison,' he said.

The 78-year-old retired judge interviewed survivors from North Korea's communist regime for an independent United Nations report he put together in 2013 and 2014.

He told Sky News anchor Andrew Bolt that Mr Warmbier was likely to have been warned about the dangers of provoking the North Korean authorities when he visited Kim Jong-Un's secretive dictatorship with a U.K. tour company in January 2016.

'They are given a very good briefing ... on the insensitivity of the country to any humorous attitudes towards its great leader,' Justice Kirby said.

'I don't think Otto was unaware but he was just a young lad with his whole life ahead of him as he thought and he really didn't take it seriously enough.'