The call for the "immediate release" of Otto Warmbier, who is in his 20s, came as Secretary of State Rex Tillerson was on his way to Japan, South Korea and China to discuss Pyongyang's drive to field a nuclear-armed missile.
"We believe his sentence of 15 years hard labor is unduly harsh for the actions that Mr. Warmbier allegedly took," said State Department spokesman Mark Toner.
"And we urge North Korea to pardon him and grant him special amnesty and immediate release on humanitarian grounds."
Toner also asked that the Swedish embassy in Pyongyang, which offers limited consular services to US nationals incarcerated in North Korea, be allowed regular access to the student.
Warmbier was handed the tough sentence on March 16 last year by North Korea's supreme court after he admitted stealing a propaganda banner from a staff area of the Pyongyang hotel where he was staying on an organized tour.
On Tuesday, the State Department again discouraged US citizens from visiting the communist country, with which it has no diplomatic relations.
In addition to Warmbier, who was a student at the University of Virginia, Korean-American pastor Kim Dong-Chul, in his sixties, was sentenced last April to 10 years of hard labor for espionage.
Pyongyang has in the past used the detention of US citizens as a bargaining tool.
Former president Bill Clinton in 2009 and then director of national intelligence James Clapper in 2014 travelled to North Korea to negotiate and bring back convicted and jailed Americans.
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