Republican candidates vowed to stop Iran from developing an atomic bomb if they became president but differed over how to do it in a debate that tested their knowledge of world hotspots.

The US economy has been the No 1 issue for the 2012 campaign, so the CBS News/National Journal debate was a rare opportunity to hear the candidates explain how they would handle the job as commander in chief.

The candidates made no major stumbles at the 90-minute gathering, but their deep differences were reflected, such as how quickly to bring US troops home from Afghanistan, whether Pakistan deserves US assistance, and if waterboarding should be brought back for terrorism suspects.

None of the eight candidates has much foreign policy experience, save for former US Ambassador to China Jon Huntsman, but they all criticized Democratic President Barack Obama, whom they want to replace in the 2012 election.

Former Massachusetts Governor Mitt Romney laid out in stark terms what he would do about Iran's alleged pursuit of nuclear weapons.

Romney said he would pursue tough economic sanctions and make clear the US would be willing to go to war to stop Iran from getting a nuclear weapon.

"Look, one thing you can know and that is if we re-elect Barack Obama, Iran will have a nuclear weapon. And if we elect Mitt Romney, if you elect me as the next president, they will not have a nuclear weapon," he said.

The UN nuclear watchdog, the International Atomic Energy Agency, reported on Tuesday that Iran appeared to have worked on designing an atomic bomb and may still be conducting secret research related to building such weapons. Iran says its nuclear ambitions are limited to peaceful electricity generation.

Businessman Herman Cain, happy to be talking about something other than sexual harassment allegations that have dogged him recently, said the only way to stop Iran from a nuclear weapon was through economic means, squeezing Tehran through sanctions and boosting Iran's opposition movement.

Former US House of Representatives Speaker Newt Gingrich, who came to Spartanburg riding a new wave of support as the conservative alternative to the more moderate Romney, declared he would launch covert operations within Iran in order to be able to deny them later.

Texas Governor Rick Perry might have been the biggest winner at the Wofford College debate simply by surviving it with no major stumbles, after he famously forgot one of the three government agencies he would eliminate at the previous debate on Wednesday.

When CBS News moderator Scott Pelley said he had thought a lot about Perry's answer that night, Perry drew laughs from the audience with the droll reply, "I have too."    

Perry was asked how he would handle nuclear-armed Pakistan, a rebellious US ally angered by the secret mission that killed al Qaeda leader Osama bin Laden last May. Perry said he would look at whether to eliminate US aid to Pakistan.

"They don't deserve our foreign aid that they're getting, because they're not being honest with us. American soldiers' lives are being put at jeopardy because of that country," Perry said.

While Gingrich said the idea "makes perfect sense," longshot candidate Rick Santorum, a former US senator, was adamantly opposed.

"Pakistan is a nuclear power," Santorum said. "We cannot be indecisive about whether Pakistan is our friend. They must be a friend."    

On Afghanistan, Romney quibbled with Obama's decision to bring thousands of US troops home from Afghanistan next September instead of December 2012, while Huntsman said flatly it was time to bring them home now in order to use the money to rebuild the US economy.

"I don't want to be nation building in Afghanistan when this nation so desperately needs to be built," Huntsman said.

There was a split over whether to reinstitute the use on terrorism suspects of waterboarding, a stimulated drowning technique that critics call torture. - Reuters