Donald Trump has threatened NATO ally Turkey with "large sanctions" over its detention of a US pastor held on suspicion of terror offences and spying.
The US president said Andrew Brunson, 50, was "suffering greatly" after spending 18 months in a Turkish jail, with health problems forcing him to be moved to house arrest earlier this week.
Mr Trump has repeatedly demanded the release of the evangelical Christian, who faces up to 15 years behind bars if found guilty of "committing crimes on behalf of terror groups without being a member".
The accusation contends that he worked to convert Kurds to Christianity to sow discord in Turkey amid a failed coup against the government two years ago.
If also convicted of espionage, Mr Brunson – who denies all charges – could receive another 20 years in jail.
Mr Trump offered his support in a tweet sent from Air Force One on Thursday, describing Mr Brunson as an "innocent man of faith".
"The United States will impose large sanctions on Turkey for their long time detainment of Pastor Andrew Brunson, a great Christian, family man and wonderful human being," he said.
"He is suffering greatly. This innocent man of faith should be released immediately!"
Vice President Mike Pence had also threatened economic sanctions earlier, telling a conference on religious freedom that the White House will respond if Turkey did not take "immediate action".
Mr Pence insisted that Mr Brunson – who lived in North Carolina until moving to Turkey 23 years ago – was "an innocent man" and that there was "no credible evidence against him".
Turkey has so far shown no sign of giving in to the calls from Washington, and its foreign minister said it would "never tolerate threats" over the detention of the pastor.
Mevlut Cavusoglu wrote on Twitter: "No-one dictates to Turkey. We will never tolerate threats from anybody. Rule of law is for everyone; no exception."
Turkish ambassador Ibrahim Kalin labelled Mr Trump's language as "unacceptable" and said the US "must reconsider its approach" so not to inflict "further damage to its own interests and its alliance with Turkey".
Any release and return to the US for Mr Brunson has previously been linked to the extradition of Turkish military coup suspect Fethullah Gulen, who is living in exile in Pennsylvania.
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President Recep Tayyip Erdogan holds him responsible for a failed bid to remove him from office in July 2016, and requests for him to be arrested and extradited have not been granted.
Mr Brunson will remain in detention pending the outcome of his trial, which has been adjourned until 12 October.