US lawmakers have written to Turkey's President Recep Tayyip Erdogan asking him to release an American missionary pastor, Andrew Brunson, held in prison and accused of terrorist activities.

Brunson and his wife Norine had been working in Turkey since 1993 and led the Resurrection Church in the coastal city of Izmir. Norine was released after 12 days of detention but Andrew Brunson was told he had been accused of terrorism by a secret informant. He has been held for four months with limited access to legal or consular help or to his family.

The letter is signed by 78 lawmakers including 37 senators, headed by the leadership of the Senate Foreign Relations Committee and the House Foreign Affairs Committee.

It argues there is no evidence to back Turkey's allegations Brunson was involved with armed terrorists. The letter says: Mr Brunson's imprisonment has been raised repeatedly by US Government officials with officials of the Government of Turkey. Unfortunately, high-level efforts to secure Mr Brunson's release have been unsuccessful. We have closely followed developments with this case, and are deeply disappointed.'

It refers to the 'close partnership' between the US and Turkey, and says: 'Now is the time for our countries to reaffirm respect for fundamental freedoms and the rule of law while reasserting our shared commitment to addressing security challenges through partnerships and cooperation.

'In this spirit of partnership, we respectfully ask you to consider Mr. Brunson,s case and how the recent treatment of Mr. Brunson places significant strain not only on him and his family, but also on the robust bilateral relationship between the United States and Turkey.'

President Erdogan's rule has been increasingly authoritarian since a failed coup last year, with tens of thousands of people arrested or dismissed from their jobs. A referendum in April is set to give him sweeping powers and entrench his political position for years.