A Canadian pastor serving a life sentence in North Korea for subversion said in an interview with CNN he spends eight hours a day digging holes at a labour camp where he has not seen any other prisoners.

Hyeon Soo Lim, a South Korean-born Canadian who was the head pastor at one of Canada's largest churches, has been held by the North since February. Lim, who was 60 at the time of his arrest, was sentenced to hard labour for life in December for attempting to overthrow the North's regime.

"I wasn't originally a laborer, so the labor was hard at first," Lim said in Korean through an interpreter. "But now I've gotten used to it."

The charges against Lim had lacked specifics, but Lim said it may have had to do with his open criticism of the North's three generations of leaders.

"I admit I've violated this government's authority, system and order," Lim said in the interview aired on Monday. Asked if his biggest crime was speaking badly of the North's leaders, he said: "Yes, I think so."

Lim, who was interviewed at a Pyongyang hotel, his hair cropped short and wearing a gray padded prison uniform bearing the number "036" on his chest, works eight hours a day, six days a week digging holes in an orchard at a labour camp where he has seen no other prisoners, CNN said.

Two stern faced guards led him into the room, one ordering him to sit, then stand, then sit again.

Lim gets three meals and day and regular medical attention, CNN said. His church has said Lim had a "very serious health problem, very high blood pressure".

The church has said he had visited the North more than 100 times since 1997 and helped set up an orphanage and nursing home. Lim, who had lived in Canada since 1986, is the only Western citizen known to be held currently in North Korea.

He has asked the authorities for a copy of the Bible but has yet to receive one, and he prays every day that North and South Korea can be unified so a situation like his will not happen again, CNN said.