WASHINGTON—The U.S. took a step toward cutting Iran off from the global economy on Thursday, levying sanctions on a financing network and accusing the country’s central bank of helping funnel U.S. dollars to the blacklisted elite military unit known as the Quds Force.

Acting jointly with the United Arab Emirates, the U.S. Treasury Department imposed sanctions on several Iranian firms, individuals and officials it said are operating an illegal currency-exchange network in the U.A.E.