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Israel has intensified its efforts to block the sale of advanced Russian air-defense missiles to Iran that would be a serious obstacle to any Israeli airstrikes against the Islamic Republic's nuclear infrastructure, according to media reports in Tel Aviv and Moscow.

Moscow has repeatedly claimed it has not delivered any of the five S-300 units it agreed to sell Iran under an $800 million contract it is said to have signed in early 2008.

S-300, considered one of the most effective all-altitude air-defense systems in the world

Some U.S. officials are concerned that Israel, "for reasons of threat perception and, strategic planning culture and operational capabilities, would feel compelled to attack Iran's nuclear facilities before the SD-300s become operational," according to Jane's Intelligence Digest.

The Americans believe that the Israelis would have some success if they launched a pre-emptive strike, although it is highly unlikely that would knock out all of Iran's nuclear facilities.