The Treasury Department said the duo -- Novin Energy Co. and Mesbah Energy Co. -- were guilty of fostering the proliferation of weapons of mass destruction (WMD).

Its action came a day after Iran informed the International Atomic Energy Agency that it planned to resume research and development into its "peaceful nuclear energy programme" next Monday.

The United States and European Union, suspecting the Iranian civil nuclear programme is a cover for developing an atomic bomb, have demanded that Iran refrain from all nuclear enrichment activities.

The Treasury said the two Iranian companies were fronts for the Atomic Energy Organization of Iran (AEOI), which it designated as a WMD proliferator in June.

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

"We will continue to expose and isolate the individuals and entities that facilitate these networks," he said in a statement.

The action against the Iranian firms was taken under a presidential executive order that bans US citizens from any transaction with a designated entity, and freezes any assets the entity may have in the United States.

The Treasury said Novin had transferred millions of dollars on the AEOI's behalf "to entities associated with Iran's nuclear programme". The company shares the same address as the AEOI, it said.

Mesbah, a state-owned company subordinate to the AEOI, "has been used to procure products for Iran's heavy-water project", the Treasury said.

Heavy water is a potential source of plutonium for nuclear weapons but "is believed to have no credible use in Iran's civilian nuclear power programme, which is based on light-water reactor technology", it said.

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