North Carolina native Andrew Brunson, 50, has been jailed since October 2016. If found guilty, he faces up to 35 years in prison.
Brunson ran a Protestant church in southwestern Turkey and has lived in the country for more than two decades.
When his trial began Monday, Brunson denied the charges. “I’ve never done something against Turkey," he said. "I love Turkey. I’ve been praying for Turkey for 25 years."
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In a letter to President Recep Tayyip Erdogan seen by NBC News, the senators describe the indictment detailing the allegations against the evangelical pastor as "an absurd collection of anonymous accusations, flights of fantasy, and random character assassination."
It suggested that Brunson was being used as a “political pawn by elements of the Turkish government bent on destroying the longstanding partnership between two great nations."
The letter added that U.S. government had been "patient in the hope that justice would be done" and Brunson would be freed. However, the 66 senators say they now have “concluded that other measures will be necessary to ensure that the government of Turkey respects the right of law-abiding citizens and employees of the United States to travel to, reside in, and work in Turkey without fear of persecution.”
The letter's signatories include Republican Sen. Thom Tillis, who attended the first day of Brunson's trial.
In September, Erdogan hinted that Brunson would be returned to the U.S. if Washington extradited Pennsylvania-based cleric Fethullah Gulen. Ankara blames Gulen for Turkey's deadly 2016 coup attempt, but he denies any involvement.
Brunson is accused of having links to supporters of Gulen.
On Thursday, Republican Sen. James Lankford and Democratic Sen. Jeanne Shaheen, who also signed the letter, said they would renew their push for sanctions targeting Turkish officials. They called the charges against Brunson "fabricated" and likened his imprisonment to "hostage-taking."
President Donald Trump earlier this week tweeted his support for Brunson, alleging that the pastor was “being persecuted in Turkey for no reason.”
Pastor Andrew Brunson, a fine gentleman and Christian leader in the United States, is on trial and being persecuted in Turkey for no reason. They call him a Spy, but I am more a Spy than he is. Hopefully he will be allowed to come home to his beautiful family where he belongs!