North Korea's supreme court sentenced American student Otto Warmbier, who was arrested while visiting the country, to 15 years of hard labor on Wednesday for crimes against the state.

Warmbier, a 21-year-old University of Virginia student, was detained in January for trying to steal an item bearing a propaganda slogan from his hotel in Pyongyang, North Korean media said previously.

"The accused confessed to the serious offense against the DPRK he had committed, pursuant to the U.S. government's hostile policy toward it, in a bid to impair the unity of its people after entering it as a tourist," the state-controlled KCNA news agency reported, using the acronym for the Democratic People's Republic of Korea.]

North Korea's sentencing of the American student was "unduly harsh," the U.S. State Department said on Wednesday as it called for his immediate release.

State Department spokesman Mark Toner urged Pyongyang to pardon Warmbier, grant him amnesty and release him immediately on humanitarian grounds. He also discouraged all U.S. citizens from traveling to North Korea.

Human Rights Watch condemned the sentence handed down to the student from Wyoming, Ohio. Japan's Kyodo news agency published a picture of Warmbier being led from the courtroom by two uniformed guards, with his head bowed, but visibly distressed.

"North Korea's sentencing of Otto Warmbier to 15 years hard labor for a college-style prank is outrageous and shocking, and should not be permitted to stand," Phil Robertson, deputy director of HRW's Asia division, said in an emailed statement.

Warmbier's defense attorney said the gravity of his crime was such that he would not be able to pay even with his death but proposed to the court a sentence reduced from the prosecution's request of a life sentence, KCNA said.

Last month, Warmbier told a media conference in Pyongyang that his crime was "very severe and pre-planned."

The U.S. State Department did not have an immediate comment and Warmbier's parents could not immediately be reached.

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A spokesman for the University of Virginia said the school was aware of the reports about the sentence and remains in touch with Warmbier's family, but declined further comment. Warmbier majors in economics with a minor in global sustainability, according to his social media profiles.

Warmbier was at the end of a five-day New Year's group tour of North Korea when he was delayed at airport immigration before being taken away by officials, according to the tour operator that had arranged the trip.

Warmbier's sentencing comes as North Korea is increasingly isolated, with the U.N. Security Council imposing tough new resolutions earlier this month following the North's January nuclear test and last month's long-range rocket launch.

North Korean leader Kim Jong Un said this week the North would soon test a nuclear warhead and ballistic missiles capable of carrying nuclear warheads, in what would be a direct violation of U.N. resolutions backed by its chief ally, China.

North Korea has a long history of detaining foreigners and has used jailed Americans in the past to extract high-profile visits from the United States, with which it has no formal diplomatic relations.

North Korea is also holding a Korean-Canadian Christian pastor it sentenced to hard labor for life in December for subversion. The North is also holding a Korean-American and three South Korean nationals.

It has previously handed down lengthy sentences to foreigners before freeing them.

In 2014, North Korea released three detained Americans.

Ohio Governor John Kasich called on North Korea to immediately release Warmbier.

"His detention was completely unjustified and the sentence North Korea imposed on him is an affront to concepts of justice. Continuing to hold him only further alienates North Korea from the international community," Kasich, who is also a Republican presidential candidate, said in a statement.

Former New Mexico Governor Bill Richardson, who had previously traveled to North Korea, met the North's ambassador to the United Nations on Tuesday to press for Warmbier's release, the New York Times reported.

"I urged the humanitarian release of Otto, and they agreed to convey our request," Richardson was quoted as saying.

While most tourists to North Korea are from China, roughly 6,000 Westerners visit annually, although the United States and Canada advise against it. Most visitors are curious about life in the reclusive state and ignore critics who say their dollars prop up a repressive system.