Turkey's response was both harsh and dismissive, calling his words "unacceptable" and a "cheap threat." Trump's promise of unspecified punishing action marks the latest deterioration in relations between Turkey and the U.S. as President Recep Tayyip Erdogan's powers expand two years after a failed coup against his government.
Trump also has praised his counterpart, saying Erdogan's leadership is "getting very high marks." The U.S has long depended on a key air base in Turkey's south, most recently to launch airstrikes against the Islamic State group.
Pastor Andrew Craig Brunson was first detained by Turkish authorities in the aftermath of the failed 2016 coup. On Wednesday, he was let out of jail after 1 1/2 years, transferred to house arrest because of "health problems," according to Turkey's official Anadolu news agency.
Trump said that was insufficient. "He is suffering greatly. This innocent man of faith should be released immediately!" Trump wrote on Twitter. The announcement of sanctions — though no details of how or when — came as the State Department was holding a three-day event promoting religious freedom. Brunson's case has become a cause for conservative Christians who form an important part of Trump's political base.
Turkey responded that Brunson's detention falls within the jurisdiction of its independent judiciary. "Rule of law is for everyone; no exception," said Foreign Minister Mevlut Cavusoglu, also via Twitter.
And an Erdogan spokesman warned the U.S. to "reconsider its approach and adopt a constructive position before inflicting further damage to its own interests and its alliance with Turkey." Vice President Mike Pence announced the threat of action at the religious freedom conference, then Trump tweeted that his government "will impose large sanctions on Turkey for their long time detainment of Pastor Andrew Brunson."
Trump could impose certain sanctions unilaterally or try to act through Congress. Senators have previously taken steps toward blocking the sale of F-35 jets to Turkey, citing Brunson's detention as an instance of Erdogan's disregard for the rule of law.
White House spokesman Hogan Gidley declined Thursday to discuss the timing of a sanctions announcement or the decision-making process. "The president was clear on Twitter today, as was the vice president, that they fully expect, the president expects and wants Pastor Brunson to be returned immediately to the United States and, if not, they can expect sanctions," Gidley said.
Brunson, 50, an evangelical Christian pastor originally from North Carolina, could face up to 35 years in prison if convicted of espionage and "committing crimes on behalf of terror groups without being a member," references to outlawed Kurdish militants and the network of a U.S-based Muslim cleric blamed for the failed coup attempt.
Brunson denies the charges. "Brunson is an innocent man, there is no credible evidence against him," Pence said in his remarks Thursday. Trump said on Twitter last week that the pastor's detention was "a total disgrace." One of Brunson's attorneys is Jay Sekulow, who also represents Trump in the federal Trump-Russia investigation.
Ties between NATO ally Turkey and the United States have been strained by other issues. Turkey recently finalized a deal to purchase Russia's long-range S-400 missile defense system, refusing to back down despite strong opposition from the U.S. and other NATO members.
The U.S. and Turkey have also clashed over American backing of Kurdish fighters in Syria who Ankara considers "terrorists." At the conference, Pence highlighted cases of what he said were religious repression in Nicaragua, Iran, North Korea, China and Myanmar. He also condemned Islamic State group violence toward religious minorities and what he described as rising anti-Semitism in Europe.
Secretary of State Mike Pompeo also spoke. He announced additional aid for a region of Iraq previously held by the Islamic State group. Pompeo said the U.S. would provide $17 million for de-mining efforts in Nineveh, an area of Iraq historically home to many of the country's religious minorities.
Erdogan has previously linked Brunson's return to the U.S. to the extradition of cleric Fethullah Gulen, the man Turkey's government holds responsible for the failed 2016 coup. Gulen, who denies orchestrating the coup attempt, lives in Pennsylvania. Turkish requests for his arrest and extradition have not been granted.
More than 77,000 people have been arrested across Turkey since the government declared a state of emergency in the failed coup's aftermath. The crackdown has targeted journalists, activists and opposition figures.
Brunson has lived in Turkey for 23 years and served as pastor of Izmir Resurrection Church, a small Protestant congregation. During a recent hearing, Brunson he rejected charges against him. "I believe in and support Turkey's territorial integrity," he told the court. "I forgive those who lie and bear false witness against me."
Brunson's case has been adjourned until Oct. 12.
Associated Press writer Cinar Kiper in Istanbul contributed to this report.