The Treasury Department named the companies as Pars Tarash (aka Pars Trash Company), Farayand Technique, Fajr Industries Group, and Mizan Machine Manufacturing Group.

The action was taken under an executive order aimed at freezing the assets of proliferators of WMDs and their supporters, the department said in a statement.

The Treasury is barring all transactions between the companies and any US person and freezes any assets the companies may have under US jurisdiction.

"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 under secretary for terrorism and financial intelligence (TFI).

Friday, the Group of Eight leading industrialized nations threatened to take "further measures" if Iran continues to refuse to comply with United Nations demands that Tehran halt its uranium enrichment.

"We again urge Iran to take the steps required by the international community, and made mandatory by these resolutions, to suspend all its enrichment-related and reprocessing activities, including research and development, and allow negotiations to begin," Britain, Canada, France, Germany, Italy, Japan, Russia and the United States, said in a final statement at the end of a summit in Germany.

The UN Security Council has already imposed two sets of sanctions targeting Iranian ballistics and nuclear industries to punish the government's defiance of the UN demand.

Iran has repeatedly denied accusations that its nuclear program is masking an atomic arms program, saying it was for civilian purposes.

US President George W. Bush issued the executive order to combat WMDs in June 2005. Since then, the Treasury has frozen the assets of 30 entities and two individuals.