If Iran eventually acquires nuclear weapons, it would be unlikely to share them with the Islamic militants it backs in the Middle East, National Security Advisor Giora Eiland said on Wednesday.
Israel has supported U.S.-led diplomatic efforts to deny Iran the means for making a bomb.
It argues that a nuclear-armed Iran would be a direct threat and could embolden allied Lebanese and Palestinian militant groups to step up attacks on Israelis.
But Eiland played down speculation among analysts and Iran's foes that Tehran might also supply proxies with portable nuclear weapons such as a radiation-spreading "dirty bomb."
"I don't think they would be ready to share this knowledge," he told foreign reporters.
"Iran is the state that supports terror more than any other state," Eiland said. "They are extreme and anti-Israeli ... but I didn't say they are not responsible."
Iranian President Mahmoud Ahmadinejad says his country's reactors are for energy only, but his calls for the elimination of Israel -- believed to hold the region's only atomic arsenal -- have stoked global fears of a nuclear confrontation.
Israel has urged United Nations Security Council sanctions on Iran so it suspends a program that includes uranium enrichment, which would be a key step in production of any nuclear weapons. Iran says it seeks only power only for energy generation.
Eiland said that sanctions, or the threat of them, would be effective only if applied within a "matter of months," and urged that Security Council powers - which have been divided on how aggressively to confront Iran - speak in "one voice."
"The Iranians have tried to present a different picture, as if they have crossed all points of no return, as if they have managed to overcome all the technological problems, so the world has to accept the fact of [their] nuclear capability," he said.
"That is not reality, not from the technological point of view nor, at this point, from the diplomatic point of view."
Western intelligence agencies have said Iran is years away from attaining the know-how to build a bomb independently.
Like the U.S., Israel has not ruled out a military strike as a last resort against Iran. But though it set a precedent by bombing the main Iraqi reactor in 1981, Israel is not widely believed to be capable of tackling Iran's more formidable facilities alone.
According to Eiland, a nuclear-armed Iran would prompt a regional arms race, making future conflicts potential catastrophes.
"If Iran, at the end of the day, manages to achieve nuclear weapons against the will of the rest of the world ... the conclusion that might be made by 1 billion Muslims over the world is that Ahmadinejad is right," he said.
"From that moment, every conflict, every crisis in the Middle East is going to take place under an Iranian nuclear umbrella," he said.