WASHINGTON (Reuters) - The Bush administration may think tough talk will discourage Iran's nuclear ambitions, but U.S. policy on Iraq and North Korea has left the Islamic state believing that only nuclear weapons can deter the possibility of U.S. invasion, experts said on Thursday.

Iran, which President Bush has branded part of an "axis of evil" along with North Korea and prewar Iraq, saw Baghdad fall to U.S.-led forces in April 2003, the same month that North Korea told the United States it possessed nuclear weapons.

Now, with 138,000 U.S. troops in Iraq and North Korean diplomatic talks promising attractive benefits for Pyongyang, Ray Takeyh of the Council on Foreign Relations said the message to Iran was clear.

"You've got to become North Korea, or you will be Iraq," said Takeyh, the council's senior fellow for Middle Eastern studies.

"Biological and chemical weapons don't deter the U.S. military and are no guarantee of territorial integrity or sovereignty," he said. "But nuclear weapons have a bargaining utility."

Added Vali Nasr, a Middle East expert who teaches at the Naval Postgraduate School: "(Iran has) come to believe, rightly or wrongly, that they're more likely to manage a threat to the regime if they have a nuclear capability."

The International Atomic Energy Agency, the U.N. nuclear watchdog, said in a new report that Iran plans to process 37 tonnes of raw uranium. That could give the country enough material for five bombs, though the IAEA found no conclusive evidence of an Iranian arms program.

Tehran insists the only purpose of its nuclear program is the peaceful generation of electricity.

The Bush administration, which accuses Iran of pursuing nuclear weapons, intends to try to persuade the IAEA board, at a meeting later this month, to find Iran is not in compliance with its nuclear Non-Proliferation Treaty obligations and to send the issue to the U.N. Security Council.

The United States, which severed diplomatic ties with Tehran after the 1979 Islamic revolution, has refused to rule out the possibility of military action against Iran.

But some experts doubt Iran can be stopped from acquiring nuclear weapons given the country's industrial development and the momentum of its nuclear program, which began in the 1970s under the U.S.-backed shah.

"The most important entity to prevent Iran from obtaining a nuclear weapons capability is the Iranian government itself," said Rand Corp. analyst John Parachini.

Army War College professor Sherifa Zuhur said the challenge of getting Iran to divulge its nuclear status will test the Bush administration's post-Sept. 11 nonproliferation policy.

"You can't begin any further process of nonproliferation unless you know where everyone is," she said.

The administration has held out Libya's voluntary disarmament as an example for Iran, while trying to encourage democratic change inside the country by supporting reformers.

But experts said Libya offers no comparison with Iran and warned that domestic politics may not offer a solution.

"This issue's viewed the same way it is in India and Pakistan. It's a source of national prestige," said Nasr.

"There are pragmatic politicians who believe this is the only issue where the regime can possibly be seen on the right side of things by an otherwise unhappy population."