The Democratic White House hopefuls vowed Wednesday to defend Israel against any Iranian attack but differed on how to engage the Islamic republic over its nuclear ambitions.

At a televised debate ahead of next Tuesday's Pennsylvania primary, Hillary Clinton and Barack Obama agreed that a nuclear-armed Iran was unacceptable.

Both called for diplomacy but Obama went further in renewing a promise of "direct talks" at a leaders' level with Tehran, along with other US foes.

Iran should be presented with "carrots and sticks," the Illinois senator said, while stressing "they should also know that I will take no options off the table when it comes to preventing them from using nuclear weapons or obtaining nuclear weapons."

"We cannot permit Iran to become a nuclear weapons power," Clinton said, ruling out any summit talks and condemning President Mahmoud Ahmadinejad for raising doubts about who really carried out the September 11 attacks of 2001.

But the former first lady said "we've got to begin diplomatic engagement with Iran" at a lower level of officialdom than Obama's preference for top-level contacts.

"And we want the region and the world to understand how serious we are about it," she said. "The (President George W.) Bush policy has failed. Iran has not been deterred."

Obama earlier pledged to Jewish voters here that he would do his utmost to help Israel fend off any regional threat, as he criticized ex-president Jimmy Carter for seeking to meet with Hamas leaders.

At the debate, he said: "An (Iranian) attack on Israel is an attack on our strongest ally in the region, one whose security we consider paramount.

"That would be an act of aggression that I would consider unacceptable and the United States would take appropriate action."

Clinton, whose New York senate seat represents a high proportion of Jewish voters, was more categorical.

"And we will let the Iranians know that, yes, an attack on Israel would trigger massive retaliation," she said.

"But so would an attack on those countries that are willing to go under the (US) security umbrella and forswear their own nuclear ambitions."