SEOUL, South Korea — Matthew Miller, an American held in North Korea since April for “hostile acts,” on Thursday began a six-year hard labor sentence he said involved farm work and isolation, media reports said.

Miller, 25, told the Associated Press he had written to U.S. First Lady Michelle Obama, Secretary of State John Kerry and his predecessor Hillary Clinton asking for help.

“Prison life is eight hours of work per day,” Miller, dressed in grey prison clothes and a hat, said in the video interview. “Mostly it’s been agriculture, like in the dirt, digging around. Other than that, it’s isolation, no contact with anyone.”

CNN also reported a North Korean government official released a photo of Miller taken on Wednesday. The picture showed him dressed in grey prison clothes with his head shaved, the number 107 on his chest and staring away from the camera.

The U.S. State Department said it was working to secure the release of all three Americans currently held in North Korea.

“Our foremost goal is to resolve these cases and secure the earliest possible release of our detained U.S. citizens,” a State Department spokesperson said, adding it was “unhelpful for us to try to negotiate with North Korea through the media.”

The United States is still willing to send Ambassador Robert King, its special envoy for North Korean human rights issues, to Pyongyang in an effort to secure their release, the spokesperson said. Pyongyang has twice rescinded invitations for King to visit at the last moment.

The State Department said on Thursday it was sending Glyn Davies, its special representative for North Korea policy to China, South Korea and Japan for meetings next week to discuss North Korea policy with senior officials.

North Korea’s state media said on Saturday Miller had pretended to have secret U.S. information and was deliberately arrested in a bid to become famous and meet Kenneth Bae, another American detainee in a North Korean prison.

Reuters reported earlier that Miller spent months in South Korea pretending to be an Englishman named Preston Somerset and invested time and money hiring artists to help create his anime adaptation of “Alice’s Adventures in Wonderland,” the Lewis Carroll fantasy with which he seemed fascinated.

Bae, a missionary of Korean descent, is serving a 15-year hard labor sentence after being convicted of crimes against the state last year.

Bae has also appeared before cameras appealing to the U.S. government for help to secure his release. He said he was being held in a special correctional facility.

A third American, Jeffrey Fowle, was arrested for leaving a copy of the Bible in the toilet of a sailor’s club in the port city of Chongjin and is currently awaiting trial.

The United States has said Pyongyang is using U.S. citizens as “pawns” to win a high-level visit from Washington.

Former U.S. presidents Jimmy Carter and Bill Clinton have both made trips to the North to secure the release of Americans held in the secretive state.