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.

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