North Korea has accused the Trump administration of exploiting the death of a US student who died soon after being released from detention in the Asian nation.

The state-run Korean Central News Agency quoted a foreign ministry spokesman as criticising the White House for using the death of Otto Warmbier for propaganda purposes against North Korea.

"The fact that the US is employing even a dead person for the conspiracy campaign to fuel the international atmosphere of putting pressure on (North Korea) shows how vile and inveterate the hostility of the US policy-makers towards (North Korea) is," the unidentified spokesman said in a statement.

Mr Warmbier, who was on holiday in North Korea, was sentenced to 15 years of hard labour in March 2016 for allegedly stealing a propaganda poster two months earlier.

North Korea has said Mr Warmbier fell into a coma that resulted from botulism and a sleeping pill.

The spokesman reiterated North Korea's denial that it tortured the American student, who was detained for more than year and died soon after his release to the US while still in a coma.

"We provided him with sincere medical care on humanitarian grounds in consideration of his failing health until he returned to the US," he said.

The statement expressed particular indignation that President Donald Trump had criticised North Korea's top leader.

"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," it said.

Mr Trump referenced Mr Warmbier's death in comments critical of North Korea during his debut speech to the UN general assembly earlier this month.

Mr Warmbier's parents told a Fox News TV show on Tuesday that North Korea tortured and "destroyed" him.

Mr Trump tweeted afterwards: "Otto was tortured beyond belief by North Korea."

The tweet added to a series of recent accusations and heated exchanges between his administration and North Korean officials.

An Ohio coroner on Wednesday said her office was unable to determine what caused the brain damage that led to Mr Warmbier's death, other than it stemmed from oxygen deprivation more than a year before his death.

"Could that have been torture at the time? We don't know," Dr Lakshmi Sammarco said.