Experts say that if Tehran acquires and operates the S-300 missile batteries it would make any strike by Israel or the United States on Iran's nuclear facilities more difficult.
Israeli defense sources said on Wednesday that Iran was set to receive the system, also known in the West as the SA-20, by the end of the year.
First delivery of the S-300 batteries was expected as soon as early September, one Israeli source said, though it could take six to 12 months for them to be deployed and operable.
But Pentagon press secretary Geoff Morrell said: "We firmly believe, based upon our understanding of the situation, that the Iranians will not be receiving that Russian anti-aircraft system this year."
Morrell, who was responding to a query from Reuters, declined to elaborate on the reasons for the Pentagon's view.
His comments expanded on remarks by Defense Secretary Robert Gates, who said on July 9 that it was "highly unlikely that those air defense missiles would be in Iranian hands any time soon."
Both Washington and Israel say they want to resolve their disputes with Tehran over its nuclear program through diplomacy, but they have not ruled out military action.
The United States, Israel and other Western nations say they believe Iran is trying to build a nuclear bomb. Iran insists its uranium enrichment program is intended only for energy generation.
A perception in financial markets that confrontation between Iran and Israel or the United States was growing more likely helped drive oil prices to record highs earlier this month.
Iran, which already has TOR-M1 surface-to-air missiles from Russia, said last December that an unspecified number of S-300s were on order. Moscow denied there was any such deal.
The best S-300 can track 100 targets at once and fire on planes 120 km (75 miles) away.
Analysts say any strike on Iran would pose significant military challenges in any case as its nuclear sites are believed to be numerous, dispersed and fortified. (Reporting by Andrew Gray; Editing by David Storey)
