"We believe that he's being held unjustly. He's gone through the criminal process, and he's been detained for, as you noted, more than a year. We believe his sentence of 15 years hard labor is unduly harsh -- harsh, rather, for the actions that Mr. Warmbier allegedly took," State Department acting spokesman Mark Toner said at a regular briefing.

"We urge North Korea to pardon him and grant him special amnesty and immediate release on humanitarian grounds," Toner said.

"We would also like to have access to him, regular access, through our protecting power, which I believe is Swedish embassy," Toner also said he strongly advises against traveling to the North.

American visitors have often been detained in North Korea on charges of anti-state and other unspecified crimes. Widespread views are that the communist nation has used the detentions as bargaining chips in its negotiations with Washington.

Kenneth Bae, a Korean-American missionary, served two years of detention in the North before being released in November 2014 when US Director of National Intelligence James Clapper made a secret trip to Pyongyang to win his release and that of another US detainee. (Yonhap)