The US Treasury Department said Novin Energy and Mesbah Energy were guilty of aiding the proliferation of weapons of mass destruction (WMD).
It said the Iranian companies were fronts for the Atomic Energy Organisation of Iran (AEOI), which the Treasury designated as a WMD proliferator in June, 2005.
"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.
Under a presidential order the Treasury can ban US citizens from any transactions with entities it designates as WMD proliferators.
It can also freeze assets that the entity may own in the United States.
According to the Treasury Department, Novin Energy had transferred millions of dollars on the AEOI's behalf "to entities associated with Iran's nuclear programme."
It also claimed the company also shares the same address as the AEOI.
The Treasury added the other company Mesbah, a state-owned subordinate to the AEOI, "has been used to procure products for Iran's heavy-water project".
Heavy water is a potential source of plutonium for nuclear weapons.
The Department said it’s suspicious because heavy water "is believed to have no credible use in Iran's civilian nuclear power programme, which is based on light-water reactor technology."
Iran to resume nuclear research
The move comes a day after Iran told the International Atomic Energy Agency that it planned to resume research and development into its “peaceful nuclear energy programme.”
The United States and the European Union suspect it’s a cover for developing an atomic bomb and have demanded Iran stand down its nuclear enrichment activities.
A delegation from Tehran is to meet with the UN’s nuclear watchdog in Vienna on Thursday to discuss Iran’s plans to resume atomic fuel research following its suspension two years ago.
Director of the International Atomic Energy Agency (IAEA) Mohamed ElBaradei wants to seek "clarifications" from the delegation about this latest move.
The IAEA will inquire if Tehran wants to remove seals the it had placed on Iranian nuclear facilities.