North Korea has sentenced an American student to 15 years of hard labour after he was arrested while visiting the repressive country.

Otto Warmbier, a University of Virginia student, was detained in January for trying to steal a banner bearing a propaganda slogan from his hotel in Pyongyang .

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

Warmbier, who is from Wyoming, Ohio, and was 21 at the time of his arrest, told a press conference in Pyongyang last month that his crime "is very severe and pre-planned."

"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.

The young man was at the end of a five-day New Year's group tour of North Korea when he was seized at airport immigration.

State media has not yet reported the sentencing, but China's Xinhua state news agency said it was handed down by the Supreme Court.

The sentencing is seen by some as overly harsh and has been linked to the worsening relations between the US and North Korea.

The secretive state 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.

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

Just before he was sentenced, diplomat and former Governor of New Mexico Bill Richardson, who had previously negotiated the release of American prisoners from the dictatorship, called for the young man to be freed.

According to the New York Times, he met North Korea's ambassador to the United Nations in New York.

He later told the paper: "I urged the humanitarian release of Otto, and they agreed to convey our request."

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

The sanctions were a response to the North conducting its fourth nuclear test and launching a satellite into space.

North Korea's leader Kim Jong-un has threatened "indiscriminate" nuclear attacks against the US and the South, and has said his country will soon test a nuclear warhead.

However analysts still doubt whether the country has the capacity to carry out a nuclear attack.