Israel would not be able to halt Iran's reported quest for atomic weapons with a single strike, a senior Israeli defense official said on Sunday.
Israel and the West suspect Iran is trying to use its nuclear program to develop atomic weapons, a charge denied by Tehran which says it wants to generate electricity.
Both Israel and the United States have hinted they might consider taking military action as a last resort to stop Iran getting the bomb.
The defense official, who in line with Israeli army guidelines declined to be identified, mentioned Iran during a review of the security situation in the Middle East in a briefing to foreign reporters.
"We're not talking about Iraq or Syria where one strike would derail a program," the official said, referring to Israel's 1981 air strike that destroyed Iraq's atomic reactor and the bombing in 2007 of a Syrian site which the U.N. atomic agency said was very likely a nuclear reactor.
"With Iran it's a different project. There is no one silver bullet you can hit and that's over," the official said.
Israeli leaders have urged the United States and other Western countries to present Tehran with a credible military threat to back up economic sanctions already in place.
The official said the United States stood a better chance of forcing Iran to change its mind over its nuclear program than Israel.
"With all respect to Israel ... the greatest fear of the (Iranian) regime is the USA. There is no question about it."
Some analysts say the likelihood of an imminent Israeli war with Iran has ebbed, thanks to the perceived success of political pressure on Tehran.
Recent Israeli estimates do not show Iran developing nuclear weapons before 2015.
Israel is widely believed to have the only nuclear arsenal in the Middle East, and Iran has accused it of hypocrisy over the issue.
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