The United States on Wednesday called for the immediate release of a 21-year-old American student who was sentenced to 15 years' hard labour in North Korea for stealing a propaganda sign from a hotel. University of Virginia student Otto Warmbier was convicted of "subversive activities" by the country's Supreme Court.

According to AFP, White House spokesperson Josh Earnest said Pyongyang was using US citizens as "pawns to pursue a political agenda". He added that Warmbier should be granted special amnesty and set free as he would not face the same charges for the offence anywhere else in the world.

Human Rights Watch said the severity of the sentence was shocking, calling it no more than a "college prank". However, the North Korean state media was more critical, saying "The accused confessed to the serious offence he had committed against the DPRK [Democratic People's Republic of Korea], pursuant to the US government's hostile policy toward it, in a bid to impair the unity of its people, after entering as a tourist."

Hostilities between the two countries, which are locked in an uneasy armistice, have been increasing since January, when North Korea conducted forbidden nuclear tests. The country also launched a satellite in February that many countries believed was an effort to bolster its nuclear arsenal. On Wednesday, US president Barack Obama issued an executive order imposing fresh sanctions on the country. This includes freezing North Korean property in the US and banning any US exports or investments in North Korea.