Washington: The United States on Tuesday imposed sanctions on 21 firms it believes are front companies for the Iranian government, stepping up 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.

The department also added two Iranian organizations and seven Iranian individuals operating throughout the Middle East to the Treasury's terrorism blacklist.

The action, which comes under an executive order, freezes any assets they may have under US jurisdiction and prohibits Americans and U.S. firms from engaging in any transactions with them.

"Today's designations expose Iran's use of its state apparatus-including the Islamic Revolutionary Guard Corps-Qods Force-and state-run social service organizations to support terrorism under the guise of providing reconstruction and economic development assistance or social services," the Treasury said.

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.

The United States previously sanctioned Iran's Revolutionary Guard entities over their support for terrorist activities and Iran's nuclear and missile programs. Iran denies Western allegations that it is seeking atomic weapons, insisting that it only wants peaceful nuclear energy.