(Adds details on firms, background)

By David Lawder

WASHINGTON, June 8 (Reuters) - The U.S. Treasury said on Friday that it has designated four more Iranian companies as proliferators of weapons of mass destruction, a move that bans U.S. firms from dealing with them and freezes any assets they may have under U.S. jurisdiction.

The Treasury department said in a statement that the four companies are owned, controlled by or act on behalf of companies and entities previously blacklisted by the Bush administration over Iran's nuclear program and missile development efforts.

The United States has stepped up commercial pressure on Iran since the United Nations in December imposed sanctions on Iran over its refusal to suspend nuclear activities the West fears are aimed at making atomic weapons.

The Treasury has prohibited U.S. dealings with Iranian state-owned Bank Sepah and Bank Saderat, a move that has also deterred many foreign banks from doing business with them.

The Treasury also has targeted firms it claims are linked to key Iranian government agencies responsible for the nuclear program and acquisition of ballistic missile technology.

Two of the firms banned in Friday's announcement, Pars Tarash and Farayand Technique, are "owned or controlled by or act or purport to act for on behalf of the Atomic Energy Organization of Iran," or AEOI, the Treasury said.

It added that AEOI is the main Iranian institute for nuclear research and development activities, including centrifuge and laser enrichment of uranium.

The other two firms, Fajr Industries Group and Mizan Machine Manufacturing Group, are linked to the Aerospace Industries Organization, an Iranian Ministry of Defense subsidiary that the Treasury said controls and manages Iran's missile programs.

The Treasury said Fajr has procured missile guidance and control equipment, including high-strength steel alloys, while in 2005, Mizan Machine purchased a state-of-the art crane capable of supporting missiles.

"An investigation of this transaction determined the crane was probably intended for use in Iran's Shahab missile program," The Treasury said in a statement.

The actions were taken under an executive order that allows the Bush administration to ban transactions with proliferators of weapons of mass destruction, and a top Treasury official said more such designations were likely.

"So long as Iran continues to pursue a nuclear program in defiance of the international community's calls to halt enrichment, we will continue to hold those responsible to account for their conduct," said Stuart Levey, Treasury undersecretary for terrorism and financial intelligence.

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