Turkish prosecutors on Friday ordered the seizure of assets belonging to an Iranian-Turk gold trader who has been a key witness in the Manhattan trial of a Turkish bank executive accused of violating United States sanctions on Iran, Turkish state media reported.

The order cited a part of the Turkish penal code that addresses the leaking of state secrets. It came a day after the witness, Reza Zarrab, testified in Federal District Court that he was told in 2012 by a Turkish official that Recep Tayyip Erdogan, then prime minister, ordered two Turkish banks to participate in a billion-dollar sanctions-busting scheme.

Mr. Zarrab, 34, entered a secret guilty plea in October to conspiring to evade the sanctions and other charges, and, in a bid to reduce his punishment, agreed to assist prosecutors. Mr. Zarrab, whose plea agreement was reported this week in court, testified on Wednesday that “cooperation was the fastest way to accept responsibility and to get out of jail at once.”