An American man recently sentenced to six years of hard labour in North Korea says he is digging in fields eight hours a day and being kept in isolation, but that so far his health isn’t deteriorating.

Under close guard and with only enough time to respond to one question, 24-year-old Matthew Miller spoke briefly to an Associated Press television journalist at a Pyongyang hotel, where he had been brought to make a phone call to his family. It was his first appearance since he was convicted on 14 September of entering the country illegally to commit espionage.

“Prison life is eight hours of work per day. Mostly it’s been agriculture, like in the dirt, digging around,” Miller said when asked if he was in prison and if so, what conditions were like.

“Other than that, it’s isolation, no contact with anyone. But I have been in good health, and no sickness or no hurts,” he said, showing little emotion.

Wearing a prison-style gray uniform and cap, Miller was filmed sitting down at a booth at the hotel and pressing the buttons on a phone while a guard stood behind him. Officials said Miller spoke to his father, but the AP journalist was not allowed to hear the conversation. Miller is gnerally not allowed phonecalls.

Miller, of Bakserfield, California, showed several letters he had written pleading for help from influential Americans, including Michelle Obama, John Kerry, and Hillary Clinton. Miller then enclosed them in a letter he posted to his family from the hotel.

At Miller’s 90-minute trial, North Korea’s supreme court said he tore up his tourist visa at Pyongyang’s airport upon arrival on 10 April and admitted to having the “wild ambition” of experiencing prison life so he could secretly investigate North Korea’s human rights situation.

Miller is one of three Americans detained in North Korea. Jeffrey Fowle, who was arrested in May for leaving a Bible at a sailor’s club, is expected to be tried in court soon. Kenneth Bae was sentenced in 2013 to 15 years of hard labour.

Last week, Robert King, the US special envoy for North Korean human rights issues, said Pyongyang has not accepted American offers to send a high-level envoy to seek the release of the three men. King said that freeing the detainees could provide a diplomatic opening in ties between the two countries, but that Washington would not give in to attempts to “extort” political gain from the detentions.

King would not specify whom the Obama administration was willing to send. But Scott Snyder, senior fellow for Korea studies at the Council on Foreign Relations thinktank, said he was told by the administration that it has offered in recent weeks to send Glyn Davies, who leads US diplomacy on North Korea’s nuclear weapons programme, and that Pyongyang had not responded favourably.

In 2009, North Korea detained two American journalists, Laura Ling and Euna Lee, who were later freed after former US president Bill Clinton visited Pyongyang. In 2011, former president Jimmy Carter visited North Korea to win the release of imprisoned American Aijalon Gomes, who had been sentenced to eight years of hard labour for crossing illegally into the country from China.