The White House has accused North Korea of jailing a 21-year-old American student for political reasons and called for his immediate release.

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

Mr Warmbier, a 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 offence against the DPRK he had committed, pursuant to the US 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.

Claiming North Korea was using US citizens as "pawns to pursue a political agenda," White House spokesman Josh Earnest called for Mr Warmbier to be freed.

"We strongly encourage the North Korean government to pardon him and grant him special amnesty and immediate release."

Human Rights Watch has also condemned the sentence handed down to the student from Wyoming, Ohio.

Japan's Kyodo news agency published a picture of Mr 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 labour 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.

Mr Warmbier's defence 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 that is reduced from the prosecution's request of a life sentence, KCNA said.

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

He 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.

Mr Warmbier's sentencing comes as North Korea is increasingly isolated, with the UN 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 this week said the North would soon test a nuclear warhead and ballistic missiles capable of carrying nuclear warheads, in what would be a direct violation of UN resolutions backed by its chief ally, China.

North Korea has a long history of detaining foreigners and has used jailed US citizens 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 labour 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 US citizens.