North Korea on Thursday accused Donald Trump of exploiting Otto Warmbier's death, calling the US president an "old lunatic" for alleging that the American student was tortured while in Pyongyang's custody.

In a statement issued by the official KCNA news agency, North Korea's foreign ministry attacked the US for "luring and pushing" the 22-year-old student into breaking the country's laws.

"Trump and his clique, for their anti-DPRK propaganda, are again exploiting the death of Otto Warmbier, an American college student who had been under reform through labour for the criminal act he committed against the DPRK and died after returning to the US," it said, using the acronym for the North's official name.

Warmbier, who was visiting the North as a tourist in January 2016 when he was arrested and imprisoned by the regime, died in June this year, just days after he was released from custody and sent home in a mysterious coma.

In the statement, the North accused "an anti-DPRK conspiracy organisation in the US" of sending Warmbier to the country on a criminal "mission".

"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," it said.

The statement came after a US medical examiner said Warmbier had shown no obvious signs of torture despite assertions by his parents and Trump.

- Accusations of torture -

His parents, in a series of American television interviews Tuesday, said their son showed signs of torture, including teeth that appeared to have been "rearranged," and hands and feet that were disfigured.

"They kidnapped Otto, they tortured him, they intentionally injured him. They are not victims, they are terrorists," Fred Warmbier said on the programme "Fox and Friends".

After the airing of the interview, Trump for the first time accused North Korean leader Kim Jong-Un's regime of torturing Warmbier.

Trump said: "Otto was tortured beyond belief by North Korea."

But coroner Lakshmi Sammarco, who examined Warmbier's body after his death, said there was no clear evidence of physical torture -- including no recently broken bones or damaged teeth.

"We don't know what happened to him. That's 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.'"

Tensions have flared in recent weeks following Pyongyang's sixth nuclear test that triggered a new round of tough United Nations sanctions and an escalated war of words between Kim and Trump, who has ignored pleas from US allies to tone down his rhetoric.

In its latest statement, the North's foreign ministry said: "Trump should... be fully aware that he and his clique shall be held totally responsible for all the undesirable turn of events to be caused by the careless and imprudent remarks that he gabbles on wagging his tongue in and out of his big mouth".

Three Americans accused of various crimes against the state remain behind bars in the North.

Source: AFP