Mr. Zarrab, who pleaded guilty and agreed to cooperate with the authorities, testified that he had also paid bribes to Zafer Caglayan, then Turkey’s economy minister, and Suleyman Aslan, the general manager of Halkbank, for assistance in carrying out the scheme.

Image Mehmet Hakan Atilla in a photo from social media.

Mr. Atilla is to be sentenced on April 11. His lawyers have asked the judge, Richard M. Berman of Federal District Court, to “temper justice with mercy.” They argued that federal guidelines call for only four to five years in prison and that Mr. Atilla should serve “significantly below” that.

But prosecutors told the judge that at a time when the United States and other nations “were engaged in the momentous undertaking of depriving the government of Iran of funding for its malign and deadly activities” — including its pursuit of nuclear weapons and ballistic missiles, and its support of terrorism — “Atilla was a key player in massively undermining those efforts.”

In seeking the stiffer penalty, the government said that Mr. Atilla’s sentence should be more than 15½ years and comparable to 20-year sentences imposed in “analogous cases.”

“Atilla’s offenses simultaneously opened a multibillion-dollar channel of illicit funding for the government of Iran and relieved crucial financial pressure on Iran during negotiations to limit its nuclear program,” the office of Geoffrey S. Berman, the United States attorney in Manhattan, wrote.