North Korea said Thursday it had released US student Otto Warmbier "on humanitarian grounds", state media reported, two days after he was evacuated from Pyongyang after falling into a coma while imprisoned in a labour camp.

"Otto Frederick Warmbier, who had been in hard labour, was sent back home on June 13, 2017 on humanitarian grounds according to the adjudication made on the same day by the Central Court of the DPRK," the state-run Korean Central News Agency said in a statement.