US officials have demanded the immediate release of the American student who was sentenced to 15 years of hard labour for stealing a propaganda banner from his Pyongyang hotel. 

The US State Department condemned 21-year-old Otto Warmbier's sentence as "unduly harsh" and White House spokesman Josh Earnest said it was politically motivated. 

Mr Earnest said it was "increasingly clear" that North Korea sought to use American citizens as pawns in their political agenda.

The University of Virginia student was arrested at the end of a five-day New Year's group tour of North Korea when he delayed at airport immigration and taken away by officials, according to the tour operator who organised the trip. 

 He was convicted on charges of subversion in a secretive trial that reportedly lasted less than an hour. 

The trial was reportedly witnessed by a representative of the Swedish embassy which looks after US affairs in the country as America does not have formal diplomatic relations with North Korea. 

The countries are still technically at war after the conflict between North and South Korea in 1950-1953 ended in an armistice rather than a peace treaty. 

State Department spokesman Mark Toner said: "We're going to remain in very close coordination with the Swedes on this matter. It's my understanding that he was in reasonable health".

He called for Warmbier to be immediately pardoned and released on humanitarian grounds. 

The state-controlled KCNA news agency said: "The accused confessed to the serious offense against [the country] 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".

His defence lawyer reportedly said the gravity of his crime was so severe that he would not be able to pay even with his death but managed to reduce the sentence from the prosecution's request of a life sentence, according to KCNA.

In a televised press conference last month, Warmbier tearfully admitted to removing the political slogan from the staff holding area of the Yanggakdo International Hotel. 

He said:  "I apologise to each and every one of the millions of the Korean people, and I beg that you see how I was used and manipulated".

He claimed he had been "lured by the United States administration to commit a crime in this country".

It comes as relations between the so-called "Hermit kingdom" and the West have are increasingly tense following its renewed nuclear tests. President Barack Obama has issued an executive order imposing "robust new sanctions" on North Korea after its Jan. 6 nuclear test and Feb. 7 rocket launch that used ballistic missile technology.

But North Korean leader Kim Jong Un said the country 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.

North Korea is currently holding a Korean-Canadian Christian pastor it sentenced to hard labour for life in December for subversion, a Korean-American and three South Koreans.

Additional reporting by Reuters

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