IZMIR, Turkey (Reuters) - The lawyer for a U.S. pastor who goes on trial in Turkey on Monday over alleged links to a group accused of orchestrating a failed military coup said the preacher had been arrested for his Christian faith.

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Ismail Cem Halavurt said pastor Andrew Brunson, detained 18 months ago, faced the “totally unfounded” charge of aiding a terrorist organization and should be freed at Monday’s hearing in the Mediterranean city of Izmir.

A Christian pastor from North Carolina living in Turkey for 23 years, Brunson has been indicted on charges of helping the group that Turkey holds responsible for the failed 2016 coup against President Tayyip Erdogan.

His trial is one of several legal cases which have strained relations between Turkey and the United States, who are also at odds over U.S. support for a Kurdish militia in northern Syria which Turkey considers a terrorist organization.

Monday’s court session will be attended by North Carolina senator Thom Tillis and U.S. envoy for religious freedom Sam Brownback.

“There is evidence that shows Brunson was arrested due to his faith,” Halavurt told Reuters on the eve of the trial, saying Brunson’s religious role had been “classified as aiding terror organizations”.

Washington has called for Brunson’s release while Erdogan suggested last year his fate could be linked to that of U.S.-based Muslim cleric Fethullah Gulen, whose extradition Ankara has repeatedly sought to face charges over the coup attempt.

“The (pastor) we have is on trial. Yours is not - he is living in Pennsylvania ... You can give him right away,” Erdogan said in September. Gulen has denied any link to the coup attempt in which more than 240 people were killed.

Izmir’s prosecutor’s office said that sufficient evidence was obtained to charge Brunson with aiding armed terrorist organizations and obtaining confidential government information for political and military espionage. The prosecution is seeking a jail sentence of up to 35 years.

A copy of Brunson’s indictment seen by Reuters accuses him of working both with Gulen’s network and the Kurdistan Workers Party (PKK) which has fought an insurgency in southeast Turkey and is designated a terrorist group by the United States and European Union.

Halavurt said he believed Brunson would ultimately be acquitted, and there was no reason for his continued detention during trial. “Our prior expectation from the hearing is ending the arrest,” he said. “We want Brunson to be freed immediately.”

At the Resurrection Church in Izmir, the congregation prayed on Sunday for Brunson, and his wife Norine read out a message in which he talked of the darkness and silence of his detention. He signed it off: “Your brother Andrew, imprisoned for God.”