WASHINGTON -- More than $2 billion allegedly held on behalf of Iran in Citigroup Inc. accounts were secretly ordered frozen last year by a federal court in Manhattan, in what appears to be the biggest seizure of Iranian assets abroad since the 1979 Islamic revolution.

The legal order, executed 18 months ago by the U.S. District Court for the Southern District of New York, is under seal and hasn't been made public. The court acted in part because of information provided by the U.S. Treasury Department.

President...