The American evangelical pastor Andrew Brunson has sent a 'heartbreaking' message to his wife, conceding that he is struggling to cope and requesting prayer as he continues to languish in a Turkish jail some 16 months after his arrest.

'I am very discouraged. Please have prayer for me,' Brunson wrote in a note to his wife, Norine, via an embassy official, according to the American Centre for Law and Justice (ACLJ) and reported in The Christian Post.

He added: 'I love you — can't handle the thought of growing old in this place, without you.'

The ACLJ, which has been assisting the pastor's family in the US, insists that Brunson is an 'innocent US citizen' who is being held as a political prisoner by the government of the Turkish President Recep Erdogan.

The law group warned that the pastor's condition is deteriorating despite being allowed to meet with the prosecutor in Turkey last week.

Brunson was arrested in October 2016 having lived in Turkey for more than two decades and led a small Protestant church in Izmir. He was accused by authorities of having links with a terror group.

It was alleged that Brunson had ties to the US-based Islamic cleric Fethullah Gulen, whom Erdogan has blamed for the failed 2016 coup against his government.

Brunson has denied any such links.

The pastor has filed a case for wrongful detention, but it is still pending, with the ACLJ warning that Brunson can be held in prison for as many as seven years without any formal charges being filed against him.

The vice chair of the congressionally-mandated US Commission on International Religious Freedom, Kristina Arriaga, was able to meet with Brunson for an hour.

She said last month: 'I have committed to Andrew Brunson that wherever I go and wherever I can speak about this topic, I will say his name. Andrew Brunson is a victim of hostage diplomacy and he needs to be released soon.'

Arriaga visited Brunson in prison in October alongside her fellow USCIRF vice chair Sandra Jolley, and explained that, apart from his wife and a consular officer, they have been the only people allowed to visit the pastor.

Arriaga recalled that, as comparedto the photos she had seen of Brunson, he appeared to have lost at least 50 pounds.

'His belt barely held up the pants that he is wearing," she said. 'He was confused and disoriented. We said our names and the first thing he said was, "I might not remember". I said, "It's OK".'

The USCIRF vice chair added that Brunson is in a 'place of psychological and physical dislocation' and does not know what the charges against him are.

Arriaga recalled: 'He asked us, "How can a NATO ally country do this to me? What are the charges filed against me? Am I going to be here for the rest of my life?"'

Last month, Brunson marked his 50th birthday in prison, writing in a letter shared by his wife: 'I am deeply grateful to all who have prayed for me. This trial — time in prison — has stretched me far more than I ever would have imagined. I have been very weak, had many doubts, felt very alone. I know that God's grace is sustaining me, even when I do not feel that grace, and I know that the prayers of God's people are surrounding me and giving strength.'

The ACLJ said: 'We will not back down. We will continue to fight this violation of justice and human rights until Pastor Andrew is released and returned home to his family in America.'