A US coroner has concluded that Otto Warmbier's death shortly after he was released by North Korea in June was the result of oxygen deprivation but it remains unclear how that occurred.

Local media in Cincinnati, Ohio, reported that the coroner also said there were no indications that Mr Warmbier had been subjected to torture, contradicting claims by his parents that he had been mistreated in captivity.

President Donald Trump has also repeated that claim, insisting in tweets that the "brutal regime" in Pyongyang had tortured Mr Warmbier "beyond belief".