U.S. Defense Secretary Leon Panetta said Wednesday that Iran must either negotiate acceptable limits on its nuclear program or face the possibility of U.S. military action to stop it from getting the bomb.

Speaking outside the city of Ashkelon in southern Israel, with an "Iron Dome" rocket defense shield as a backdrop, Panetta sought to balance his threat to possibly use force against Iran with a message of patience. He said every measure short of military action must be exhausted first, and he suggested Israel agrees.

His Israeli counterpart, Ehud Barak, sought to portray the allies as united on Iran. But he left no doubt that his government believes time is running short to stop Iran from getting a nuclear weapon it could use on the Jewish state.

Standing beside Panetta and flanked by American and Israeli flags, Barak said he sees an "extremely low" probability that international economic sanctions will prove sufficient to cause the Iranian leaders to give up their nuclear ambitions.

"We have clearly something to lose" while the world waits for sanctions to have the desired effect, "because the Iranians are moving forward" with production of enriched uranium that could be used to build a bomb, Barak said.

Iran says its nuclear work is for civilian energy uses, but suspicions that the Islamic republic will use enriched uranium for nuclear weapons have resulted in international sanctions and saber-rattling from Israel, which perceives a nuclear Iran as an existential threat. The United States has discouraged Israel from a unilateral, pre-emptive military strike on Iran.

Panetta said repeatedly that "all options," including military force, are on the table to stop Iran, should sanctions and diplomacy _ the preferred means of persuasion _ ultimately fail.

He said he still hopes Iran will see that negotiations are the best way out of this crisis.

However, Panetta said, "If they continue and if they proceed with a nuclear weapon, ... we have options that we are prepared to implement to ensure that that does not happen."

Panetta later met with Israeli Prime Minister Benjamin Netanhayu, who has said repeatedly that if necessary he will order military action against Iran even if Washington objects. Panetta said in his appearance with Barak that he understands that Israel must make such important decisions on its own terms.

"Their effort to decide what is in their national security interest is something that must be left up to the Israelis," Panetta said.

The Panetta visit to Israeli comes just days after U.S. Republican presidential hopeful Mitt Romney met with top Israeli officials about Iran and other issues. Romney has accused the Obama administration of being too soft on Iran and not providing sufficient support to Israel.

In greeting Panetta Wednesday at Israeli defense headquarters, Barak said, "The defense ties between Israel and the United States are stronger and tighter than they have ever been and the credit now has to go, most of it, to you, Leon."

Panetta responded: "We are a friend, we are a partner, we have, as the defense minister has pointed out, probably the strongest U.S.-Israel defense relationship that we have had in history. What we are doing, working together, is an indication not only of our friendship but of our alliance to work together to try to preserve peace in the future."

Netanyahu told Israeli Channel 2 TV on Tuesday that despite reservations about an Iranian attack among former Israeli security officials and Israel's current army chief, the country's political leadership would make the final decision on any attack.

"I see an ayatollah regime that declares what it has championed: to destroy us," Netanyahu said. "It's working to destroy us, it's preparing nuclear weapons to destroy us. ... If it is up to me, I won't let that happen."

With "matters that have to do with our destiny, with our very existence, we do not put our faith in the hands of others, even our best of friends," Netanyahu said, hinting that Israel might act alone despite American misgivings.

Netanyahu said both Romney and Obama have said "Israel has the right to defend itself."

The trip to Ashkelon on Wednesday gave Panetta a chance to inspect and get briefed on an Israeli air defense system known as Iron Dome. It is designed to shoot down short-range rockets and artillery shells such as those that have been fired into the Jewish state in recent years from Islamic militants linked to Iran and based in southern Lebanon and the Gaza Strip.

Obama last week announced he was releasing an additional $70 million in military aid for Israel, a previously announced move that appeared timed to upstage Romney's trip to Israel. The stepped-up U.S. aid, first announced in May, will go to help Israel expand production of the Iron Dome system.

The Panetta visit to Israel comes at a critical time, with the U.S. considering more direct involvement in Syria's civil war and weighing its course on Iran.

Panetta acknowledged Monday that international sanctions have not pressured Tehran to give up its nuclear ambitions. But the Obama administration thinks tougher sanctions eventually will compel Iran to submit and it doesn't want Israel to attack prematurely.

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Associated Press writers Amy Teibel and Daniel Estrin in contributed to this report.

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Follow Robert Burns on Twitter: http://twitter.com/robertburnsAP