Republican White House hopefuls told Israel's supporters Tuesday that they would take tougher steps than US President Barack Obama to prevent a nuclear-armed Iran.

Frontrunner Mitt Romney suggested he would be more willing than Obama to consider using military force while Rick Santorum backed an ultimatum demanding Iran stop nuclear production to avoid action by the US to "tear down" its facilities.

Newt Gingrich, a long-shot for president, told the Washington gathering he would back everything short of war to "undermine and replace" the government of President Mahmoud Ahmadinejad.

Criticizing what he sees as an Obama administration policy weighted toward engagement, Romney told the powerful pro-Israel lobby he would "bring the current policy of procrastination toward Iran to an end" if he is elected president.

"Hope is not a foreign policy," Romney told the American Israel Public Affairs Committee via video link from Columbus, Ohio, adding the only thing "respected by thugs and tyrants... is our power and our readiness to use it."

He added: "I will not delay imposing further crippling sanctions. I will not hesitate to fully implement the ones we already have. I'll make sure that Iran knows of the very real peril that awaits it if it becomes nuclear."

"I will engage Iran's neighbors, I will station multiple aircraft carriers and warships at Iran's door," he said.

Romney told thousands of AIPAC delegates, who gave him strong applause, that "as president, I will be ready to engage in diplomacy, but I will be just as ready to engage our military might."

Obama, while refusing to rule out military action, has ratcheted up sanctions on Iran, including measures targeting the vital oil and banking industries, in a bid to force Iran back to talks over its nuclear program.

The European Union announced Tuesday that the United States, Russia, China, France, Britain and Germany have offered to resume the long-stalled talks with Tehran over its nuclear drive, though no date or venue has been set.

Unlike Obama, who has only spoken of preventing Iran from obtaining a nuclear weapon, Romney said: "We must not allow Iran to have the bomb or the capacity to make a bomb."

Santorum spoke from the AIPAC podium to take a harsher line on both Obama and Iran, caling Obama's policy "reticent."

"He says he has Israel's back... From everything I've seen from the conduct of this administration he has turned his back on the people of Israel," Santorum said.

"We need to do more than talk," he said, adding there is "a need to set forth an ultimatum."

"We need to say to the Iranian government, the time is now. You will stop your nuclear production now," he said to roaring applause, stronger than when Obama spoke to the same audience on Sunday.

"You will open up your facilities for inspectors from the United States and other countries so we can certify that those efforts are stopping and being dismantled now," Santorum said.

"We need to be prepared if that ultimatum is not met, to engage Prime Minister (Benjamin) Netanyahu and the people of Israel in an effort to make sure that if they do not tear down those facilities, we will tear them down ourselves," he said to strong applause.

Using a video-link, Gingrich said he would "initiate a strategy... to undermine and replace the Iranian dictatorship by every possible method short of war" to bring in a government the US can deal with.

Drawing strong applause, he said he would require no advance notice from Israel if it decided to take "pre-emptive measures" to avoid the threat of a nuclear holocaust.

Iran denies all charges it seeks atomic weapons, saying its program is exclusively for peaceful purposes.

Prior to the Republican speeches, US Democratic Senator Carl Levin warned AIPAC delegates against those who might use the issue of how to provide security to Israel, the top US ally in the Middle East, for partisan gain.