An American pastor detained for 18 months in Turkey went on trial Monday on terrorism and espionage charges, with prosecutors demanding a 35-year prison sentence. The case has become a focal point in U.S.-Turkish tensions.According to local media reports, Andrew Brunson broke down in tears after being told by the court in the Aegean city of Izmir that he would have to remain incarcerated. News reports said the decision was based on evidence from witnesses that Brunson might pose a flight risk. Brunson has been held in solitary confinement, which he said has been affecting him psychologically. He pleaded to the court for a change in his prison conditions."I want the whole truth to be revealed. I reject all the accusations in the indictment. I haven't been involved in any illegal activity," 50-year-old pastor Andrew Brunson told the court in the Aegean city of Izmir. "I haven't done anything against Turkey. On the contrary, I love Turkey. I have been praying for Turkey for 25 years."The Brunson case is drawing strong criticism from the United States. Washington has repeatedly called for his release, including a demand from U.S. President Donald Trump to "expeditiously" return the pastor.U.S. Senator Thom Tillis and the U.S. envoy for religious freedom, Sam Brownback, attended Monday's proceedings. "The United States cares deeply about our relationship with Turkey," Brownback told reporters during a recess at the trial. "That relationship is going to have difficulty moving forward as long as Andrew Brunson is incarcerated."Brunson is accused of aiding an armed terror organization called FETO, which is blamed for a failed 2016 coup. Prosecutors allege FETO's head is the U.S.-based Turkish cleric Fethullah Gulen — a charge he denies.Brunson has lived in Turkey for 23 years and, with his wife and two sons, runs a church in Izmir.Political considerationsIn March, Tillis said Brunson had become a "political pawn" in U.S.-Turkish relations.Ankara has become infuriated over the failure by the United States to extradite Gulen, who lives in self-imposed exile in Pennsylvania. Washington maintains that the issue is a matter for the courts.Last year, Turkish President Recep Tayyip Erdogan appeared to make an explicit offer. "Give us the pastor [Brunson] back, they say. You [U.S.] have one pastor [Gulen] as well. Give him [Gulen] to us," he said. "Then we will try him [Brunson] and give him to you." A presidential adviser subsequently insisted the president was only speaking rhetorically.Experts, however, suggest Brunson is a victim of a new, aggressive Turkish foreign policy."Erdogan is using this [Brunson detention] as a political kidnapping," said international relations professor Huseyin Bagci of Ankara's Middle East Technical University,"Tayyip Erdogan, in this case, does not follow any principle. If he sees this is in the interest of his political agenda, he can release him [Brunson], no problem. We have the prime example of the German journalist Deniz Yucel. On television, Erdogan said, 'This man will never leave Turkey as long as I am president.' Now Yucel is as free as a bird, living in Germany," Bagci added. Yucel, like Brunson, was arrested on terrorism charges and held for more than a year, but was released from pre-trial detention in February. According to German media reports, Yucel's release was in exchange for a series of concessions by Berlin, including arms sales, a charge the German government denied.Carrot or stick?Brunson is not the only U.S. citizen detained in controversial circumstances.In February, NASA scientist Serkan Golge was jailed for seven years on terrorism offenses. Two local U.S. diplomatic employees are also being held in Turkey on terrorism charges."There is a lot of anger in the Congress, in the Pentagon, and the State Department toward Turkey," said political columnist Semih Idiz of Al-Monitor website. "The release of Pastor Brunson would let out a lot of pressure. I understand Brunson comes from the same church as the newly nominated secretary of state [Mike Pompeo]. Obviously, this pastor will remain as a big thorn in Turkish-U.S. relations."Washington's support of the Syrian YPG Kurdish militia in its war against Islamic State continues to anger Ankara, which accuses the group of being linked to an insurgency inside Turkey. Erdogan's deepening relationship with his Russian counterpart, Vladimir Putin, is drawing growing U.S. criticism, especially over Turkey's purchase of an S-400 missile system from Russia.Congress has reportedly been considering introducing sanctions against Ankara. The Brunson case, analysts warn, is likely to give further impetus to those moves."The continued incarceration of Pastor Brunson will raise the odds for sanctions," said political analyst Atilla Yesilada of Global Source Partners.Experts, however, suggest a deal is ready to be made. "It depends on what the Americans are offering. I would not be surprised if he [Brunson] was released," Bagci said.