The United States imposed sanctions on 21 firms it believes are front companies for the Iranian government. The step has increased pressure on the Islamic republic over its nuclear program.Accusing Tehran of trying to dodge sanctions by setting up opaque trading companies, the Treasury Department named a host of banking, mining and other companies spread throughout Europe and Japan as government-backed."As its isolation from the international financial and commercial systems increases, the government of Iran will continue efforts to evade sanctions," said Treasury undersecretary Stuart Levey. Those moves, he said, included "using government-owned entities around the world that are not easily identifiable as Iranian to facilitate transactions in support of their illicit activities."The firms included two Belarus-based banks, two Germany-based investment firms, and mining and engineering companies in Japan, Germany, Luxembourg, Italy and Iran.American citizens and businesses will now be barred from doing business with the proscribed companies.It is the latest of a series of US moves to force a halt to Iran's nuclear program, which Western governments say is a cover for a nuclear weapons drive.