Matthew Miller's trial is reported to have lasted just 90 minutes. Photo: BBC

Matthew Miller was arrested in April, shortly after arriving as a tourist.

The US accuses North Korea of using Mr Miller and two other detained Americans as pawns in a diplomatic game.

The North Korean authorities have not specified the charges against Mr Miller, but they claim he tore up his visa and demanded asylum.

During the trial, prosecutors said Mr Miller admitted having a "wild ambition" to spend time in a North Korean prison so he could find out about the country's human rights situation, the Associated Press (AP) reports.

Notes produced in court also suggested he had become a fugitive because he was involved with Wikileaks, the organisation that has leaked US state secrets.

Our correspondent in Seoul, Steve Evans, says it is impossible to know how those notes were written - whether under duress or not - and it is not clear whether there is any truth to the allegations.

After a 90-minute trial, the sentence was handed down and Mr Miller was handcuffed and led from the room, AP reports.

The White House has described securing the release of Mr Miller and the two other American citizens detained in North Korea as a "top priority".

Mr Miller was arrested shortly after he arrived in North Korea in April

In the past the US has been able to negotiate the release of American detainees.

Notably two journalists who were held whilst filming a documentary in North Korea were granted a "special pardon" after former President Clinton travelled to the country.

The US has offered several times to send Robert King, its special envoy for North Korean human rights issues, to Pyongyang to discuss the detainees, but these visits have been cancelled by North Korea.

North Korean 'shelter'

Mr Miller, 24, of Bakersfield in California, had been in custody since 10 April.

Little information has been released about him, and the US State Department said this was partly because he had not signed a Privacy Act Waiver, which allows information about him to be released to the public.

According to KCNA, Mr Miller tore up his tourist visa on arrival in the country and shouted that "he came to the DPRK [North Korea] after choosing it as a shelter."

In a brief interview with CNN earlier this month, attended by North Korean officials, Mr Miller said: "I will say that I prepared to violate the law of the DPRK before coming here."

He also said he deliberately committed his "crime", although he did not specify what he had done wrong.

Accordingly, much mystery remains, our correspondent says.

Human pawns

In a recent interview with Associated Press, all three American detainees appealed to the US government to send a high-ranking representative to negotiate about their freedom.

American tour guide and missionary Kenneth Bae is serving a 15 year sentence in a labour camp in North Korea

Jeffrey Fowle was arrested for allegedly leaving a Bible at a provincial club

State department official Daniel Russel told Reuters last week that the US found North Korean treatment of its citizens "objectionable and distressing".

"This is the way that they play," he said. "They use human beings, and in this case Americans citizens, as pawns."

Jeffrey Fowle came to North Korea as a tourist but was arrested in May for allegedly leaving a Bible in a public place. North Korea considers the distribution or spreading of Christian information as incendiary.

Korean-American missionary Kenneth Bae, who was arrested in November 2012, is serving 15 years in a labour camp after being convicted of trying to overthrow North Korea's government.