Thousands have signed a petition calling for the release of a Canadian pastor sentenced to life imprisonment in North Korea.

The campaign to free Hyeon Soo Lim, a Canadian-Korean pastor in his 60s, now has over 128,000 signatures and calls for his immediate release. The petition on change.org criticises Canadian Prime Minister Justin Trudeau for failing to do more.

"The Canadian government needs to express more than mere hope and dismay," the petition read.

Lim was sentenced to a life of hard labour on December 16, 2015 after at least 10 months in detention. He was charged with 'harming the dignity of the supreme leader,' 'trying to use religion to destroy North Korea' and 'assisting North Koreans escape.' The pastor pleaded guilty to the charges.

"Since 1997... Lim has been supporting where help is most needed, including an orphanage, public schools, and a nursing home in North Korea," the petition read.

"He's made more than 100 trips to North Korea during the last 20 years, supplying food, textbooks, and agricultural tools to civilians. His trips were not political in nature."

Very little is known about conditions in North Korea although persecution charity Open Doors estimates at least 70,000 Christians are imprisoned for their faith.

"Christians face arrest, torture, imprisonment and death for daring to believe there is a higher authority than the nation's leader, Kim Jong-un," the charity claims.

"Many follow Jesus in secret; parents often don't even tell their children about their faith for fear they will be revealed."

For the last fourteen years North Korea has topped Open Doors' World Watch List for the worst countries to live as a Christian.

Although the new Canadian Prime Minister has expressed his "tremedous" concern at the sentence, there has been little evidence of pressure being applied on the dictatorial state.

"We certainly hope to be able to engage with this individual and stand up for his rights," Trudeau said. Diana Khaddaj, a spokeswoman for Canada's Global Affairs Department, said Canada is "dismayed at the unduly harsh sentence... particularly given his age and fragile health."

The petition's authors say this isn't strong enough.

"How will Prime Minister Trudeau and United Nations Secretary General Ban Ki-Moon respond to a humanitarian imprisoned - and most likely abused and tortured - in the world's most atrocious regime?" the petition asks.