"We have ongoing concerns about the prospective sale of such weapons to Iran and other countries of concern," Scott Stanzel, a White House spokesman, said from Crawford, Texas, where President George W. Bush was spending the yearend holiday on his ranch.
Iranian Defense Minister Mostafa Mohammad Najjar announced Wednesday that Russia would deliver the system, but said the date for the delivery would be unveiled later. Moscow has not confirmed the sale.
The announcement came as Washington is seeking to impose new United Nations sanctions on Iran over its refusal to halt its nuclear program, which the United States fears could be used to build an atomic bomb.
Earlier this year Moscow frustrated Washington by delivering to Tehran 29 TOR-M1 air defense missile systems, in a deal estimated to be worth 700 million dollars.
Iranian state media touted the S-300 as an even more sophisticated system than the TOR-M1, saying it could hit incoming enemy targets at a greater altitude. Iran said in January it had successfully test fired the TOR-M1.
The United States had urged Russia to cancel that sale, saying it was a mistake when the UN Security Council had imposed sanctions on Iran’s ballistic missile industry as part of measures against its nuclear drive.