A US citizen serving a prison sentence in North Korea has given insight on life inside dictator Kim Jong-un's harsh penal system.

Matthew Todd Miller, 24, has just started a six year jail term for committing a crime inside the Stalinist state.

The American revealed he spends all day digging in fields inside the nuclear-armed communist state and that he is being kept completely isolated, so that he sees only his captors.

"Prison life is eight hours of work per day. Mostly it's been agriculture, like in the dirt, digging around," Miller said when asked what conditions were like in prison.

"Other than that, it's isolation, no contact with anyone. But I have been in good health, and no sickness or no hurts."

Miller was able to reveal details of his ordeal only by snatching a few words with a Press Association reporter at a Pyongyang hotel to which he had been whisked to make one phone call to his family.

The Californian has been jailed for unspecified crimes, which he has admitted to committing in previous statements. But it is hard to verify the claims, given North Korea's record on human rights and treatment of prisoners in the country's sprawling prison system.

According to North Korean media, which acts as the mouthpiece for the ruling regime, Miller had wanted to emulate Edward Snowden, the spy-turned-leaker who revealed the extent of surveillance by western governments on citizens, often with the assistance of large tech companies.

Korean Central News Agency said Miller had tried to infiltrate prisons in the Hermit Kingdom and called him "rudely behaved."

"He perpetrated the above-said acts in the hope of becoming a world famous guy and the second Snowden through intentional hooliganism," it said.

Speaking earlier this month, Miller told CNN he was guilty of the offences which landed him behind bars, saying: "[I] prepared to violate the law of DPRK before coming here. And I deliberately committed my crime."

North Korea holds a total of three US citizens in prison, including Kenneth Bae and Jeffrey Fowle.