The President has today acted to block all official Iranian assets in the United States, including deposits in United States banks and their foreign branches and subsidiaries. This order is in response to reports that the Government of Iran is about to withdraw its funds. The purpose of this order is to ensure that claims on Iran by the United States and its citizens are provided for in an orderly manner.
The order does not affect accounts of persons other than the Government of Iran, the Central Bank of Iran, and other controlled entities. The precise amounts involved cannot be ascertained at this time, but there is no reason for disturbance in the foreign exchange or other markets.
The President is taking this action pursuant to the International Emergency Economic Powers Act, which grants the President authority "to deal with any unusual and extraordinary threat to the national security, foreign policy, or economy of the United States."
Jimmy Carter, Blocking Iranian Government Property Announcement on the President's Action. Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/248918