The relationship between United States and Turkey is likely to come under pressure as long as a U.S. pastor remains in jail, a U.S. official responsible for international religious freedom said on Monday.

A Turkish court on Monday began hearing evidence against Andrew Brunson, a Christian pastor from North Carolina, on charges of links to the group that orchestrated a 2016 failed coup in Turkey. Brunson, who has lived in Turkey for more than 20 years, has denied the charges.

Speaking to reporters during a recess at the trial,Sam Brownback, the U.S. Ambassador-at-Large for International Religious Freedom said:  “The United States cares deeply about our relationship with Turkey.  But that relationship is going to have difficulty moving forward as long as Andrew Brunson is incarcerated.”

US officials are calling on Turkey  to release Andrew Brunson.