Businessman Reza Zarrab arrives at a police center in Istanbul on Dec. 17, 2013, in a case separate from the charges he now faces in the United States. Zarrab is expected to testify Wednesday at a federal trial in New York, after prosecutors said he is now a cooperating witness in an Iran sanctions case that has inflamed tensions between Turkey and the United States. (Ozan Kose/AFP/Getty Images)

A Turkish Iranian gold dealer is expected to testify Wednesday in federal court in Manhattan, the star witness in what was to have been his own trial on charges of fraud and violating sanctions. Now, he will give evidence against others in a case that is likely to further strain relations between the U.S. and Turkish governments.

After weeks of secrecy concerning the whereabouts and intentions of Reza Zarrab, prosecutors said Tuesday what many had long suspected: Zarrab had flipped, becoming a prosecution witness against one of his alleged co-
conspirators, Mehmet Hakan Atilla.

Given Zarrab’s cooperation, Atilla is now the only defendant on trial in the case. The charges against Atilla stem from his work as the deputy chief executive of Halkbank, which is majority-owned by the Turkish government.

During opening statements in the trial Tuesday, Assistant U.S. Attorney David Denton said Zarrab had pleaded guilty and would testify against Atilla, whose alleged lies “blew a billion-dollar hole in the U.S. economic sanctions against Iran.’’

The prosecutor said Atilla designed a “massively successful’’ scheme to disguise U.S.-dollar transactions on behalf of the Iranian government.

Zarrab is described by U.S. authorities as well connected to senior members of the Turkish government, which in turn calls his prosecution an attack on its country. Diplomatic experts anticipate that, as the case wears on, the rift between Washington and Ankara will only widen.

Turkish President Recep Tay­yip Erdogan has personally lobbied the U.S. government to release Zarrab and has linked the case to a 2016 coup attempt against him, claiming that the same forces behind the putsch — which he blames on a cleric named Fethullah Gulen — are behind the sanctions case against Zarrab.

This month, after it became clear that Zarrab might be cooperating with U.S. investigators, Turkey’s foreign minister called him a “hostage’’ being forced to testify against the Turkish government.

Turkish prosecutors announced Nov. 18 that they had launched their own investigation into two of the American prosecutors who have overseen the Zarrab case to determine whether evidence was illegally obtained — remarks that U.S. Attorney Joon H. Kim, one of the officials Turkey claims to be investigating, called “ridiculous on their face.”

Opening statements Tuesday from prosecutors and defense lawyers made clear that Zarrab’s credibility would be a central point of contention — and that alleged corruption in the Turkish government would be discussed as part of the prosecution’s case.

Prosecutors said the sanctions-busting scheme was uncovered by police in Turkey, leading to corruption charges in that country in 2013. The case collapsed after an internal purge led to some investigators being sent to prison. Zarrab was arrested, but the charges were ultimately dropped.

“Bribes got rid of the case,’’ Denton said, but they “could not get rid of the evidence,” including wiretapped conversations involving Zarrab. Now, prosecutors said, Zarrab will explain to jurors exactly how the scheme worked.

Atilla’s lawyer Victor Rocco tried to convince jurors from the outset that Zarrab’s account could not be trusted. “Zarrab hopes he can buy freedom, a shortcut back to his lavish life with the rich and famous,” Rocco said. “He’s a liar, a cheat, a corrupter of men.’’

The lawyer said Zarrab was cooperating only to save himself, and possibly to remain in the United States and join the government’s witness protection program.