WASHINGTON/ISTANBUL (Reuters) - U.S. President Donald Trump on Thursday threatened to slap “large sanctions” on Turkey unless it freed an American pastor, prompting an angry response from Ankara and further escalating tensions between the two NATO allies.

A spokesman for Turkish President Tayyip Erdogan called Washington’s threats unacceptable and damaging to the U.S.-Turkey alliance.

“The United States must reconsider its approach and adopt a constructive position before inflicting further damage to its own interests and its alliance with Turkey,” Erdogan’s spokesman, Ibrahim Kalin, said in a written statement.

Turkish Foreign Minister Mevlut Cavusoglu called U.S. Secretary of State Mike Pompeo to discuss the threat of sanctions, Turkish and State Department officials said, without elaborating.

The Trump administration escalated the pressure campaign to free Andrew Brunson a day after a court ordered that he be moved to house arrest after 21 months in detention. He is on trial for terrorism charges.

“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,” Trump wrote in a tweet.

“He is suffering greatly. This innocent man of faith should be released immediately!”

Shortly before, Vice President Mike Pence, a devout Christian, made a similar threat directly at Turkey’s president.

“To President (Tayyip) Erdogan and the Turkish government, I have a message on behalf of the president of the United States of America: release Pastor Andrew Brunson now or be prepared to face the consequences,” Pence told a religious freedom conference hosted by the U.S. State Department.

Neither Trump nor Pence elaborated on what sanctions they could impose, although sanctions experts speculated that the administration could consider visa restrictions against senior Turkish officials.

U.S. evangelicals have been an influential part of Trump’s coalition of support, both in his campaign and in his administration, and have pushed the White House to act on the Brunson case.

Trump’s ambassador for International Religious Freedom, Sam Brownback, said Brunson’s detention had been raised with Turkey’s leadership for some months, and while his transfer to house arrest was an improvement, “he is not free.”

The threat of sanctions prompted a downturn in Turkey’s financial markets. There was no immediate comment from U.S. lawmakers who have threatened to withhold weapons sales to Turkey as strains have escalated over Brunson’s detention.

U.S. President Donald Trump participates in a roundtable discussion on workforce development at Northeast Iowa Community College in Peosta, Iowa, U.S., July 26, 2018. REUTERS/Joshua Roberts

Turkey's lira currency briefly weakened to 4.89 to the dollar following reports of Pence's comments, but recovered later somewhat. The Istanbul bourse .XU100 fell as much as 1.75 percent before picking up slightly.

Trump’s threat comes as the Turkish lira has lost one-fifth of its value this year over concerns about the central bank’s ability to rein in double-digit inflation.

“Turkey’s economy is very shaky,” said Anders Aslund, a senior fellow at the Atlantic Council in Washington. “Erdogan has taken command over the economy and he has negative macroeconomic credibility, so we expect a serious current account crisis. U.S. threats of sanctions aggravate it.”

Brunson, who is from North Carolina and has worked in Turkey for more than 20 years, has been accused of helping the group Ankara says was behind a failed military coup in 2016. The pastor, who has denied the charges, faces up to 35 years in jail if found guilty.

“I think that the President is being wise and applying pressure,” said Senator Thom Tillis, a North Carolina Republican, who attended Brunson’s hearing and co-sponsored a bill to punish Turkey for detaining the pastor.

Erdogan has in the past linked Brunson’s fate with that of the U.S.-based Muslim cleric Fethullah Gulen. Ankara accuses Gulen of masterminding the failed coup, and has long sought his extradition to Turkey. Gulen has denied any role in the coup attempt, in which 250 people were killed.

Brunson’s detention has deepened a rift between Washington and Ankara, which are also at odds over the Syrian war and Turkey’s plan to buy missile defenses from Russia.

On Wednesday, Brunson was escorted out of prison in the coastal city of Izmir, and left in a convoy of cars. His lawyer said he was ordered to wear an electronic monitoring bracelet.

U.S. and Turkish officials had been working on a deal that would lead to Brunson’s release, and Washington had expected him to be freed at his trial last week, a source in the United States familiar with developments told Reuters on Wednesday.

U.S. officials had been under the impression that the deal was in place, the source said, adding that when Brunson was not released, Pence spoke with Trump and the two agreed harsh new policy measures were needed to force the issue.

Addressing Brunson’s daughter in the audience, Pence said: “I promise you, as I told your father yesterday, President Trump and I will continue to fight to secure your father’s full release until he is restored to his family and returns to the United States of America.”