As one of the worst persecutors of Christians in the world, believers there are frequently detained, tortured and sent to labour camps because of their faith.
According to Release, entire families are known to have been imprisoned because one member of the family was found to be a Christian or in possession of a Bible.
A former prisoner, identified only as Mr Kim for security reasons, told Release: "If there is a hell in this world, then it is the North Korean concentration camps. [In the camps] there is only fear, and absolutely no connection to the outside world.
"The food crisis is bad in North Korea, but it is even worse in the camps. People die of malnutrition. If anyone was found praying they would disappear."
Mr Sung, another defector whose full name has been withheld for security reasons, told Release that life in North Korea was about "survival".
"We cannot say it is about human life. There is no freedom to speak," said Sung.
Mr Sung managed to escape to China, where he was later joined by his mother and brother. When in 2001 his mother and brother attempted to return to North Korea because of the insecurity of their situation in China, they were discovered by soldiers and beaten to death.
Sung witnessed their murders from the other side of the river they had just crossed to re-enter North Korea.
"I have forgiven those who killed my mother and brother, but I cannot help but feel the guilt of losing her," he said.
Sung's parents, four brothers and a sister have all been killed by the North Korean authorities. Only he and his youngest sister are still alive.
Chief Executive of Release International, Andy Dipper, said he was deeply concerned by the accounts coming from North Korean defectors.
"We are urging the government of North Korea to allow Christians to worship freely without the threat of persecution," he said.
"Release also asks Christians in the UK to pray and take action to help their persecuted brothers and sisters – and to support North Korean Christians now trying to adjust to life outside.
"Please pray that the petition will make a difference to the lives of Christians living in North Korea, whether believing in secret, or suffering in prison for their faith."