A U.S. air strike on Iran's nuclear bomb factories would set the terror nation's project back four years, much longer than if the less capable Israeli air force attacked, according to experts on Iran's national security.

But it might also kick Iran's nuclear project into high gear and escalate military retaliation in the area, especially against Israel, and cost U.S. taxpayers millions to fund the thousands of bombing strikes that will likely be needed to sideline Tehran's rush for the bomb, according to two experts associated with the Truman National Security Project, a group whose board features former Secretary of State Madeleine Albright, former Sen. Gary Hart and Center for American Progress Chairman John Podesta.

Jim Walsh, an MIT scholar who co-authored a report on bombing Iran, said the country hasn't yet started building a weapon and is instead still focused on nuclear enrichment. Striking now, he said, "would delay the program up to two-four years." He said it would be stalled for a "shorter period" if Israel hit Iran.

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His report, "Weighing Benefits and Costs of Military Action Against Iran," said other forms of warfare such as a cyber attack and covert operations, would also be required.

But in a call with reporters Truman expert Joe Costa added that there is time for diplomacy to work with Iran. "This is not the time to have heated rhetoric or calls for military action," he said.

At the United Nations this week, Iranian President Mahmoud Ahmadinejad again promised to "eliminate" Israel.