An American student who died days after being released from North Korea in a coma displayed no obvious signs of torture despite assertions by his parents and US President Donald Trump, a medical examiner said.

Otto Warmbier's death on June 19 from a lack of oxygen and blood to the brain resulted from an unknown injury that occurred more than a year before he died, Dr Lakshmi Sammarco said at a news conference on Wednesday.

"We don't know what happened to him and that's the bottom line," Sammarco said. "We're never going to know unless the people who were there come forward and say, 'This is what happened to Otto.'"

READ MORE: Otto Warmbier suffered brain damage in N Korea

The University of Virginia student fell into a coma shortly after he was sentenced to 15 years of hard labour by North Korea in January 2016. He was released 18 months into his sentence on June 12.

The coroner and a September 11 report by her office cited complications of chronic deficiency of oxygen and blood supply to the brain in Warmbier's death. Only an external examination of the body, rather than a full autopsy, was conducted at the request of Warmbier's family.

North Korea had blamed botulism and the ingestion of a sleeping pill for Otto Warmbier's health problems and dismissed torture claims.

The native of Wyoming, Ohio, was convicted of trying to steal a propaganda poster from a Pyongyang hotel and was arrested at the capital's airport.

Wednesday's revelations came a day after Warmbier's parents and Trump accused North Korea of torturing the young man.

Fred Warmbier told Fox News in an interview on Tuesday, "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."

After the interview aired, Trump for the first time accused North Korean leader Kim Jong-un's country of torturing Warmbier.

"Great interview on @foxandfriends with the parents of Otto Warmbier: 1994 - 2017. Otto was tortured beyond belief by North Korea," President Donald Trump said on Twitter.

Great interview on @foxandfriends with the parents of Otto Warmbier: 1994 - 2017. Otto was tortured beyond belief by North Korea. — Donald J. Trump (@realDonaldTrump) September 26, 2017

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

Sammarco said the Warmbiers' TV interviews had prompted her to publicly reveal her findings.

"They're grieving parents. I can't really make comments on their perceptions," she said.

North Korea on Thursday accused Trump of exploiting Warmbier's death, referring to the US president as an "old lunatic" for alleging the 22-year-old was tortured while in Pyongyang's custody.

In a statement issued by the state-run KCNA news agency, North Korea's foreign ministry accused the US of "luring and pushing" Warmbier 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.

Three Americans accused of various crimes against the state are behind bars in North Korea, which is engaged in a tense standoff with the Trump administration over its missile and nuclear weapons programmes.