ANKARA (Reuters) - A Turkish prosecutor on Tuesday sought a life sentence for a U.S. pastor over alleged links to a failed coup attempt in 2016, Dogan news agency reported, a move that threatens to undermine efforts to mend bilateral ties.

Andrew Brunson, a Christian pastor from North Carolina, U.S. who has been in jail in Turkey since December 2016, is seen in this undated picture taken in Izmir, Turkey. Depo Photos via REUTERS ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. NO RESALES. NO ARCHIVE. TURKEY OUT. NO COMMERCIAL OR EDITORIAL SALES IN TURKEY.

Washington believes Andrew Brunson, who has been in jail since December 2016, is one of several Americans unjustly detained in Turkey. During a visit to Ankara last month, former U.S. Secretary of State Rex Tillerson called for him to be freed.

The Christian pastor from North Carolina has been living in Turkey for 23 years and running a church, according to an online petition seeking his release.

The prosecutor charged him with being an “executive” of the group that Ankara holds responsible for the failed coup, the news agency said. Turkey blames the network of Fethullah Gulen, a U.S.-based Muslim cleric who denies all involvement.

In September, President Tayyip Erdogan suggested Turkey could free Brunson if the United States handed over Gulen.

It was not immediately clear what charges were brought against the pastor in the indictment. The prosecutor’s office could not be reached for comment.

Brunson was accused of obtaining secret documents with the aim of political or military espionage, and attempting to overthrow the government, Dogan and other Turkish media reported when he was initially questioned more than a year ago.

The accusations were based on the testimony of a secret witness, who said Brunson was in close contact with Gulen’s network and receiving funds from it for missionary activities.

Brunson denied the allegations, saying he was a defender of Jesus Christ and would never support any Islamic movement, Dogan reported at the time.

‘ABSURD’

The U.S. and Turkey have been at odds over several issues, notably Washington’s support in Syria for the YPG, a Kurdish militant group deemed terrorist by Turkey, and the status of Gulen.

Tillerson said during his official visit that Washington had “serious concerns” about cases against U.S. citizens arrested under the state of emergency in place in Turkey since the coup attempt, and said Brunson and others should be freed.

Following Tillerson’s visit, the two countries formed joint working groups to mend their relations.

Brunson’s daughter on Friday told the United Nations Human Rights Council that the terror allegations against her father were “absurd,” according to the European Centre for Law and Justice, a group supporting his case.

“Turkey should not get away with holding my father one more day,” Jacqueline Brunson said, urging the U.N. to help get him released.

“I know the allegations against my father are absurd. He is ...a peaceful pastor.”