WASHINGTON - US Defense Secretary Leon Panetta vowed on Tuesday that the United States would take military action to prevent Iran from acquiring a nuclear weapon if diplomacy fails."Military action is the last alternative when all else fails," he told an annual policy conference of the biggest US pro-Israel lobbying group, the American Israel Public Affairs Committee (AIPAC). "But make no mistake, when all else fails, we will act."Jpost.com staff contributed to this report.Just before Panetta spoke, three Republicans seeking their party's nomination to run against President Barack Obama in the Nov. 6 election practically outbid each other in showing their support for the Jewish state before the same audience.The frontrunner in the Republican contest, Mitt Romney, accused Obama of being soft on Iran in a videocast to the AIPAC conference and said that as US president he would be "just as ready to engage our military might" as he would be ready to engage in diplomacy.Panetta, a former director of the US Central Intelligence Agency, ticked off examples of Obama's use of force to support US national security interests, including having sent Special Operations forces hundreds of miles into Pakistan to kill al-Qaida leader Osama bin Laden in May."As the President has made clear, the United States does not bluff," the Pentagon chief said. "In this town, it's easy to talk tough. Acting tough is a hell of a lot more important."Israeli Prime Minister Benjamin Netanyahu assured Obama on Monday at the White House that Israel has not taken any decision on attacking Iran's nuclear sites, sources close to the talks said, though Netanyahu gave no sign of backing away from possible military action.The European Union's foreign policy chief Catherine Ashton said on Tuesday that six world powers have accepted an Iranian offer for talks on its disputed nuclear program after a year's standstill that has increased fears of a slide into a new Middle East war.Obama told the AIPAC lobbying group on Sunday that he viewed as legitimate Israel's fear of a nuclear-armed Iran and Israel's right "to defend itself, by itself, against any threat."In a meeting with the Israeli PM on Monday, Obama also said that all options remain on the table regarding Iran.The commander of US forces in the region, Marine Corps General James Mattis, testified to the Senate Armed Services Committee on Tuesday that Iran made him concerned about US moves to cut its overall naval force."The stacked Iranian threats... of ballistic missiles, long range rockets, mines, small boats and cruise missiles and submarines demand stronger naval presence and capability to protect vital sea lines of communications," Mattis said in written testimony on the Pentagon's fiscal 2013 budget request.US Senate Armed Services Committee Chairman Carl Levin told the AIPAC audience that Israeli missile defense systems such as Arrow, Patriot and Iron Dome, all developed with US help, would give Israel strong protection if Iran retaliated for an Israeli strike."Should Iran make the fateful choice to develop a nuclear weapon, because of the missile defenses in Israel, Iran's missiles will not stop Israel or us from doing what we have to do to prevent them from succeeding," he said."Should the sword need to be drawn to prevent Iran from getting a nuclear weapon, Israel will wield a powerful shield against Iranian retaliation - a shield strengthened and participated in by the United States of America," Levin said.