The U.S. State Department urged North Korea Tuesday to release an American student who was sentenced last year to 15 years of hard labor for subversion. In a department briefing, spokesperson Mark Toner called for the "immediate release" of 22-year-old Otto Fredrick Warmbier, describing the sentence as "unduly harsh."

Read: American Hostages In North Korea Otto Warmbier And Kim Dong Chul Threatened By Kim Jong Un Regime

"Our concern about his welfare is very well known," Toner told reporters in a conference call after a question from NBC News.

"We believe that he’s being held unjustly. He’s gone through the criminal process and he’s been detained for ... 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. And we urge North Korea to pardon him and grant him special amnesty and immediate release on humanitarian grounds. We would also like to see our – or have, rather, access to him – regular access through our protecting power, which I believe is the Swedish embassy.

"More broadly speaking, I just have to reiterate ... that we strongly discourage any travel by any U.S. citizens to North Korea given how they are treated—Mr. Warmbier’s case only being the most recent one. We urge any U.S. citizen considering travel to North Korea to visit our website, travel.state.gov, and to heed the warning there against traveling to North Korea."

Toner's comments came ahead of Secretary of State Rex Tillerson's trip to Japan, South Korea and China to discuss Pyongyang’s heightening nuclear threat.

Warmbier, a University of Virginia student, was sentenced March 16, 2016, by North Korea's supreme court for stealing a propaganda banner bearing the name of Kim Jong-il—the state’s former leader —from Yanggakdo International Hotel in Pyongyang, where he was staying as part of an organized tour.

The banner read (translation): “Let’s arm ourselves strongly with Kim Jong-il patriotism!"

Warmbier was charged with subversion under Article 60 of North Korea’s criminal code, with the court accusing him of committing a crime “pursuant to the U.S. government’s hostile policy toward (the North), in a bid to impair the unity of its people after entering it as a tourist.”

North Korea has had tense relations with the U.S. and South Korea for several years and has detained foreign civilians for unspecified reasons. Korean-American pastor Kim Dong-Chul, who was 62 at the time, was sentenced to 10 years of hard labor for espionage in April 2016.

Former President Bill Clinton and director of national intelligence James Clapper have both visited the communist nation to negotiate and bring back convicted Americans.