North Korea has accused US President Donald Trump of exploiting the death of a US student jailed in Pyongyang, describing the Mr Trump as an "old lunatic".

Otto Warmbier died days after being medically evacuated from North Korea in June, where he had been jailed for stealing a sign from a hotel.

On Tuesday, President Trump claimed the 22-year-old had been "tortured beyond belief" by the reclusive regime after the student's parents said their son was "on his deathbed when he came home to us".

Hitting back, North Korea's foreign ministry has accused the US of "luring and pushing" Otto Warmbier into breaking the country's rules, adding he was given a "mission" by "an anti-DPRK conspiracy organisation in the US".

"Trump and his clique, for their anti-DPRK propaganda, are again exploiting the death of Otto Warmbier," it said.

"The fact that the old lunatic Trump and his riff-raff slandered the sacred dignity of our supreme leadership, using bogus data full of falsehood and fabrications, only serves to redouble the surging hatred of our army and people towards the US and their will to retaliate thousand-fold."

Washington is "employing even a dead person for the conspiracy campaign to fuel the international atmosphere of putting pressure on the DPRK", it added.

US student Otto Warmbier | File photo

In a statement made before his trial, Otto Warmbier told reporters in Pyongyang he wanted the sign "as a trophy" for the mother of a friend.

A fellow churchgoer allegedly offered the student a car worth US$10,000 (€8,486) if he successfully returned to the US with the political slogan.

In its first statement on his death, Pyongyang claimed Mr Warmbier fell into a coma after suffering from botulism.

Coroner Dr Lakshmi Sammarco said there was no evidence of this, but also refuted claims he was tortured.

"We don't know what happened to him and this is the bottom line," she said.

"We're never going to know, unless the people who were there come forward and say 'this is what happened to Otto.'"

Fred and Cindy Warmbier said that when Otto was returned home he had a shaved head, was blind, deaf and "staring blankly into space, jerking violently", "howling" and making an "involuntary, inhuman sound".

Mr Warmbier told Fox News "it looked like someone had taken a pair of pliers and rearranged his bottom teeth".

Speaking on Wednesday, Dr Sammarco said she was "surprised" by the remarks as "there was no evidence of trauma to the teeth".

She added: "They're grieving parents, I can't really make comments on their perceptions."

The coroner's report said Otto Warmbier's body appeared "well-nourished" and was in a surprisingly good condition for someone who had reportedly been bedridden for more than a year - indicating he received regular care.

The student suffered brain-damaging oxygen deprivation through "an unknown insult more than a year prior to death", it added.