The Bush administration acted Wednesday to tighten the financial clamps on two Iranian companies suspected of helping to proliferate weapons of mass destruction through involvement in the country's nuclear program.

The action taken by the Treasury Department's Office of Foreign Assets Control targets Novin Energy Co. and Mesbah Energy Co.

The department ordered U.S. banks to freeze any bank accounts or other financial assets found in the United States that belong to these two companies. The U.S. government also is prohibiting Americans from doing business with the two entities.

Treasury's sanctions come one day after the United States warned Iran against pursuing new nuclear research.

Iran on Tuesday said it planned to resume nuclear fuel research after a 2 1/2 year hiatus. It wasn't specific about what research or development activities it would undertake, but said the work would be unrelated to nuclear fuel production. That fanned fresh concerns in the West that Tehran was trying to build an atomic weapon.

Novin and Mesbah are believed to be owned or controlled or acting on behalf of the Atomic Energy Organization of Iran, which the U.S. government designated in late June of 2005 as being a proliferator of weapons of mass destruction, the Treasury Department said.

A presidential executive order issued at that time provided the United States with the power to take Wednesday's action against the two Iranian companies.

Novin allegedly transferred "millions of dollars" on behalf of the Atomic Energy Organization of Iran "to entities associated with Iran's nuclear program," the Treasury Department said. Novin operates within the agency and shares the same address , the department said.

Mesbah "has been used to procure products for Iran's heavy water project," Treasury said. "Heavy water is essential for Iran's heavy-water-moderated reactor project, which will provide Iran a potential source of plutonium well-suited for nuclear weapons," the department alleged. "Heavy water is believed to have no credible use in Iran's civilian nuclear power program," it added.

"Identifying and designating supporters of WMD proliferation disrupts the networks that are vital to illicit weapons programs," said Stuart Levey, Treasury's under secretary for terrorism and financial intelligence.