The U.S. Treasury Department has designated 11 companies as fronts for an Iranian bank that Washington says helps spread weapons of mass destruction.

The Treasury said Tuesday U.S. citizens are prohibited from any transactions involving the companies, which are located in Iran, the Cayman Islands and Dubai.

The United States accuses the companies of funneling money for Bank Melli, which U.S. officials call a "known proliferator" of weapons of mass destruction.

The United States has supported three rounds of United Nations sanctions on Iran for its disputed nuclear program. It also has taken unilateral action against various Iranian entities.