U.S. Vice President Joe Biden vowed Monday that the United States will not back down from its pledge to use military action to thwart Iran's nuclear program should all other options fail.

"Iran threatens not just Israel, but the world," Biden said. "But we understand that if we're wrong our existence isn't threatened, but that if Israel is wrong it is a clear and present threat to its existence."

"President Barack Obama is not bluffing," Biden said. "We are not looking for war. We are looking to and ready to negotiate peacefully, but all options, including military force, are on the table."

In a prelude to Obama's upcoming trip to Israel — his first as president — Biden told the American Israel Public Affairs Committee that the U.S. does not want a war with Iran, but that the window for diplomacy is closing. He said prevention, not containment, is the only outcome the U.S. will accept.

Praising Obama, Biden told the crowd: "I've worked with eight presidents during my career, and I can promise before you that no president has done more for Israel's security than President Obama."

But in a sign the U.S. is still reluctant to embroil itself in another Mideast military effort, Biden cautioned more than 13,000 Israel supporters at AIPAC's annual conference that if Israel or the U.S. acts too hastily, without exhausting every other reasonable option, they could risk losing the backing of the international community.

"That matters because God forbid we have to act, it's important that the rest of the world is with us," Biden said to muted applause.

Prime Minister Benjamin Netanyahu, who couldn't attend the conference due to ongoing efforts to form a government, spoke to the audience by video link from Jerusalem. The prime minister pushed back against such reluctance, reflecting the tension still present between the U.S. and its closest Mideast ally as they seek a united front to stave off Iran's nuclear program.

"From the bottom of my heart and from the clarity of my brain, words alone will not stop Iran. Sanctions alone will not stop Iran," Netanyahu said.

"Words alone will not stop Iran. Sanctions alone will not stop Iran. Sanctions must be coupled with a clear and credible military threat if diplomacy and sanctions fail," the prime minister said to loud cheers.

Netanyahu said Iran had not yet crossed a "red line" he set at the United Nations in September, when he said Tehran should not be allowed to amass enough medium-enriched uranium that, if purified further, would be enough to power a single warhead. He gave a rough deadline at the time of spring or summer 2013.

But he told AIPAC: "Iran is getting closer to that red line and it's putting itself in a position to cross that line very quickly once it decides to do so." However, Netanyahu stopped short of any explicit threat of Israeli military action. A senior Israeli official said that while the Netanyahu government had hoped for a tougher line at the negotiations by the so-called P5+1 — made up of the United States, China, France, Russia, Britain and Germany — it was resigned to awaiting the results of the next round of talks.

Defense Minister Ehud Barak, also in attendance, dedicated much of his speech to the Iranian threat, which he described as "the greatest challenge for Israel, the region and the world."

Barak reiterated that all options must remain on the table: "We expect everyone who says so to mean it. And ladies and gentlemen — we mean it."

The defense minister went on to lay out his vision for the Israel and the Middle East, saying, "The two-state solution is the only long-term solution there is," and called the Palestinians a "difficult peace partner."

Barak also praised Netanyahu for taking "brave steps" toward restarting the peace process. "I know that a final status solution is probably not attainable today," he said, "but if that's the case — and only making an effort will help determine if this is the case — we must try reaching a fair and logical interim agreement."

If an interim agreement isn't possible, Barak told the audience, Israel "must consider unilateral measures."

The White House sees Obama's visit later in March to Israel, Jordan and the Palestinian territories as an opportunity to reset relations with Netanyahu, who has frequently bumped heads with Obama, and to strengthen ties with Israelis, many of whom bristled at Obama's early efforts to reach out to the Arab world and his decision not to visit Israel during his first term as president.

White House officials say Obama will not be bringing an ambitious new peace plan when he travels to Jerusalem, but improving relations with Israel and its leader could open the door to a stepped up U.S. effort to facilitate negotiations between Israelis and Palestinians.

"We will continue to oppose any efforts to establish a state of Palestine through unilateral actions," Biden said, referring to Palestinian efforts to seek U.N. recognition that the United States has staunchly opposed. "There is no shortcut to peace."

Netanyahu said that "Peace must be anchored in reality and security. Every time we have retreated we've received terror. We can't let that happen again. We are ready for compromise, but we will never compromise Israel's security."

Obama spoke at the AIPAC conference last year while running for re-election, but this year sent Biden, who vowed a nimble and resolute U.S. response to fluctuating threats in Iran, Syria and Egypt. He cast a vote of confidence in incoming Secretary of State John Kerry — "You're going to be happy with Kerry" — and said only through engagement would the U.S. navigate the challenges the Arab Spring presents.

Biden made no mention of the newest member of Obama's second-term national security team, Defense Secretary Chuck Hagel, whom opponents cast as insufficiently supportive of Israel. Hagel also apologized for saying the "Jewish lobby" intimidates members of the Senate into favoring actions contrary to U.S. interests.

Speaking on a panel shortly before Biden's address, Republican Sen. John McCain, a leading foe of Hagel's, said, "We need members of the national security team who are pro-Israel, not anti-Israel."

McCain also concluded that "the latest efforts at conciliation and some kind of agreement with the Iranians have failed. It's very clear that they are on the path to having a nuclear weapon."

Critical for Hagel's success in winning Senate confirmation was the fact that AIPAC never publicly took a stand on the nomination.

On the Syrian front, Biden reiterated that Syrian President Bashar al-Assad must be ousted, but made clear that uncertainty about elements of the Syrian opposition is still keeping the U.S. from arming the rebels. His comments came days after the U.S. announced a substantial shift in policy to provide non-lethal aid directly to rebels battling Assad.

"We are not signing up for one murderer's gang replacing another in Damascus," Biden said.

Netanyahu also made clear Israel's concern about where Syria's stockpile of chemical weapons and other advanced arms might end up in the midst of civil war.

"As the Syrian regime collapses, the danger of these weapons falling into the hands of terrorist groups is very real. Terror groups such as Hezbollah and al-Qaida are trying to seize these weapons as we speak," he said. "We have a common interest in preventing them from obtaining these deadly weapons."

Biden expressed a similar degree of skepticism about Egypt, where U.S. hopes that Islamist President Mohammed Morsi could play a powerful leadership role in the Mideast were dampened after anti-Semitic remarks Morsi made in 2010 were publicized and political protesters have questioned his commitment to democracy.

"We're not looking at what's happening in Egypt through rose-colored glasses," Biden said. "Our eyes are wide open. We have no illusions about the challenges that we face, but we also know this: There's no legitimate alternative at this point to engagement."

His comment came just one day after Kerry announced the release of $250 million in U.S. aid to Egypt.