The US has called on North Korea to immediately release an American student sentenced to 15 years' hard labour for crimes against the state.

Otto Warmbier, 21, was arrested for trying to steal a propaganda sign from a hotel while on a visit in January.

A White House spokesman accused North Koreans of using US citizens as "pawns to pursue a political agenda".

North Korean state media said Warmbier was convicted under an article of the criminal code relating to subversion.

The verdict was handed down by the Supreme Court.

Warmbier's trial reportedly lasted an hour

"We strongly encourage the North Korean government to pardon him and grant him special amnesty and immediate release," White House spokesman Josh Earnest said.

North Korea sometimes uses the detention of foreigners as a means of exerting pressure on its adversaries.

The BBC's Stephen Evans in South Korea says the 15-year sentence is high compared to those given to foreigners in the past.

This could be due to the elevated tensions at the moment between North Korea and the US, he says.

'Worst mistake of my life'

Warmbier, a student at the University of Virginia, was arrested on 2 January at the airport as he tried to leave North Korea. He was accused of committing "hostile acts".

He later appeared on state TV apparently confessing and saying a church group had asked him to bring back a "trophy" from his trip.

At the time, state news agency KCNA said had gone to North Korea "to destroy the country's unity" and that he had been "manipulated" by the US government.

At the end of February, at a tearful press conference in Pyongyang, Warmbier said he had "committed the crime of taking down a political slogan from the staff holding area of the Yanggakdo International Hotel".

"The aim of my task was to harm the motivation and work ethic of the Korean people. This was a very foolish aim," he was quoted as saying.

He said it was the "worst mistake" of his life.

North Korea detainees often recant their confessions once out of the country.

US tourism to North Korea is legal but the US State Department strongly advises against it.

The sentencing comes a day after veteran US diplomat Bill Richardson met North Korean officials at the UN in New York to try to push for Warmbier's release.

Mr Richardson has previously been involved in negotiations to secure the release of Americans from North Korea detention.

North Korea has ramped up its hostile rhetoric in recent weeks, after the UN imposed some of its toughest ever sanctions.