North Korea has accused Prime Minister Justin Trudeau and other officials of "recklessly spouting rubbish" in reaction to a sentence of hard labour for life given to Hyeon Soo Lim, a Canadian Christian convicted of subversion.

Lim, a South Korean-born Canadian who was the head pastor at one of Canada's largest churches, had been held by North Korea since February. He was sentenced on Wednesday in a ruling Canada called "unduly harsh."

North Korea's highest court said Lim had attempted to overthrow the government and undermine its social system with "religious activities" for the past 18 years, China's official Xinhua news agency reported.

Trudeau said after the ruling: "The issues of North Korea's governance and judicial system are well known. We are very concerned about someone being sentenced to life in North Korea."

North Korea said Tuesday the pastor was "arrested while being involved in a thrice-cursed subversion plot" against the country.

"The prime minister and other official figures of Canada are recklessly spouting rubbish against the DPRK over the judgment of its Supreme Court made by giving the penalty of life hard labor to Rim Hyon Su [sic], a Canadian citizen, on Dec. 16," an unnamed spokesman for the North Korea's Foreign Ministry said in a statement issued through the country's mission to the United Nations.

Participants pray for Canadian pastor Hyeon Soo Lim who is being held in North Korea during a joint multi-cultural prayer meeting at Light Korean Presbyterian Church in Toronto on Dec. 20. (Hyungwon Kang/Reuters)

The North Korean official said Canada should have apologized instead of criticizing the communist country.

"The Canadian side's unreasonable and uncouth response would only render the situation more complicated," the spokesman said.

"Now, the Korean people are demanding heavier punishment on Rim, hardly repressing their indignation at his crimes," he said.

Lim's 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 has lived in Canada since 1986, had a "very serious health problem, very high blood pressure", his church said. He was 60 at the time of capture.

He is the only Western citizen known to be held currently in North Korea.