An American man sentenced to hard labour in North Korea starts his six-year prison sentence today.

Matthew Miller was convicted on September 14 of "acts hostile" to North. The hermit country's Supreme Court stated during his trial that he had torn up his tourist visa after arriving at Pyongyang airport on April 10 so that he could be sent to prison and uncover the human rights situation there.

The state-run Korean Central News Agency reported: "He perpetrated the above-said acts in the hope of becoming a world famous guy and the second Snowden through intentional hooliganism."

The 24-year-old, from Bakersfield, California, was allowed to speak briefly to an Associated Press Television News journalist at a hotel in Pyongyang this week.

He told the reporter he was working eight hours a day in prison.

"Mostly it's been agriculture, like in the dirt, digging around. Other than that, it's isolation, no contact with anyone."

Miller, who was dressed in a grey shirt and cap, added that he was in good health, with "no sickness or no hurts".

He had been taken to the hotel to make a phone call home to his family and was closely supervised by a guard.

He showed several handwritten letters, including one addressed to US first lady Michelle Obama, which were then posted from the hotel. The other letters were addressed to US Secretary of State John Kerry and his predecessor Hillary Clinton.

In the letters, Miller appealed for help from the US to secure his release.

"I had been requesting help from my government and wrote a letter to the president, yet received no reply and was sent to prison for a sentence of six years," he wrote in his letter to the first lady.

"I now directly write this letter in the hopes that you can become involved in my case and help to release me from this situation."

Two other US citizens are detained in North Korea – Kenneth Bae, who is serving a 15-year hard labour sentence, and Jeffrey Fowle, who has been held since his arrest in May for leaving a Bible in a hotel room.

Bae operated tours in North Korea but the country claims his company was a front for Christian evangelical missions.

He was recently quoted by the pro-North Korean newspaper, Choson Sinbo, as saying that he felt "abandoned by the United States government".

Fowle's trial is expected to begin soon. He told the Associated Press last month that he was "desperate" to get back to his wife and three children in his native Miamisburg, Ohio.

"Within a month I could be sharing a jail cell with Ken Bae," he told AP.