A pastor imprisoned in Iran's notorious Evin Prison has spoken of the comfort he draws from knowing God is near him in his suffering.

Farshid Fathi was among the nearly two dozen Christians rounded up by authorities in Tehran in 2010.

He was sentenced to six years in prison in 2012 after being found guilty of "acting against national security through membership of a Christian organisation, collection of funds, propaganda against the Islamic Regime by helping spread Christianity in the country".

He has had to endure harsh treatment behind bars, with reporting from Voice of the Martyrs that he had to spend the first year in solitary confinement and had his foot and toe broken when he tried to help another inmate.

He is not due to be released until December 2016 but a new letter released through Elam Ministries reveals he has not lost his faith in God.

"In each of our lives, there are nights: nights of loneliness, nights of nightmare, nights of fever and pain, endless nights of prison, nights of missing, nights of hard-breathing and some scary, unexplainable nights in our lives," Fathi wrote.

He continued: "When a child has a bad night, his father or his mother lies beside him during the night, tenderly stroking his forehead, kissing his cheeks over and over, calling his name in the most lovely voice and they keep looking at the child's face until that bad night passes over. Then the child opens his eyes and thinks his father has just come to him. He was not able to sense his father's breath while he was battling with his nightmares."

He concluded: "He was not only there in the morning, but He was with us all through the dark nights of our life ... So I am absolutely sure that finally 'the sun of justice will rise with healing in its wings (Malachi 4:2).'"