A Canadian pastor serving a life sentence in North Korea for subversion said he spends eight hours a day digging holes at a labour camp, while a naturalised American citizen said he is being held by the state for spying, CNN reported from Pyongyang.

If confirmed, Kim Dong Chul, who CNN said was 60 and formerly of Fairfax, Virginia, would be the second Western citizen known to be held currently in North Korea. He was being held for spying for South Korea and asked the South or the US government to rescue him, CNN said.

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 last 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 labourer, so the labour was hard at first,’ Lim told CNN in Korean through an interpreter. ‘But now I've gotten used to it.’

The charges against Lim lacked specifics, but he said it may be related to 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 was brought into a Pyongyang hotel for the interview, his hair cropped short and wearing a grey padded prison uniform bearing the number ‘036’ on his chest. He said 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.

Lim, who had lived in Canada since 1986, 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.’

Lim had visited the North more than 100 times since 1997 and helped set up an orphanage and nursing home, according to the church.

In a separate interview, Kim told CNN he spied on behalf of ‘South Korean conservative elements’ and was arrested in October.

‘I was tasked with taking photos of military secrets and scandalous scenes,’ Kim said.

The US embassy in Seoul said it was aware of the report but did not have further comment.

A US State Department official declined to comment on the reports, saying that speaking publicly about specific cases of detained Americans can complicate efforts to get them released.

If confirmed, Kim would be the first American to be detained since the North released three US citizens in 2014.

He said he had moved to the Chinese city of Yanji near the border with North Korea and worked in the North Korean city of Rason in a trading business, when a number of South Koreans approached him

‘They asked me to help destroy the (North's) system and spread propaganda against the government,’ he said. He was being held at a Pyongyang hotel and was in good health, CNN said.