Vice President Joe Biden addresses the American-Israeli Public Affairs Committee (AIPAC) 2013 Policy Conference, Monday, March 4, 2013, at the Walter E. Washington Convention Center in Washington. AP Photo/Susan Walsh Vice President Joe Biden gets a hug from Israeli Defense Minister Ehud Barak before he addressed the American-Israeli Public Affairs Committee (AIPAC) 2013 Policy Conference, Monday, March 4, 2013, at the Walter E. Washington Convention Center in Washington. Vice President Joe Biden gestures as he addresses the American-Israeli Public Affairs Committee (AIPAC) 2013 Policy Conference, Monday, March 4, 2013, at the Walter E. Washington Convention Center in Washington. WASHINGTON (AP) - Seeking to reassure anxious Israelis and their American supporters, Vice President Joe Biden vowed Monday that the United States won't back down from its pledge to use military action to thwart Iran's nuclear program should all other options fail. "President Barack Obama is not bluffing," he said. In a prelude to Obama's upcoming trip to Israel - his first as president - Biden told a powerful pro-Israel lobby that the U.S. doesn't 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. 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 the American Israel Public Affairs Committee'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. Israeli Prime Minister Benjamin Netanyahu, speaking to the conference by video link from Jerusalem, 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. The U.S. and world nations have imposed crippling sanctions on Iran's oil and financial industries in hopes of forcing Tehran back to the negotiating table and persuading it to give up nuclear ambitions. Netanyahu has warned that the world has until this summer - at the latest - to keep Iran from building a bomb, and has repeatedly hinted at Israel's willingness to launch a military strike to stop it, an endeavor the United States likely would be dragged into. Iran insists that program is intended for peaceful purposes such as power generation and medical uses, but the head of the U.N. nuclear agency said this month he cannot guarantee all of Iran's activities are peaceful unless Tehran increases cooperation. The White House sees Obama's visit later in March to Israel, Jordan and the Israeli-occupied 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 won't 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. Obama spoke to pro-Israel group's 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. Days after the U.S. announced a substantial shift in policy in Syria to provide non-lethal aid directly to rebels battling President Bashar Assad, Biden reiterated that 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. "We are not signing up for one murderer's gang replacing another in Damascus," Biden said. 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. Greeting Israeli Defense Minister Ehud Barak with all the warmth of a lifelong friend, Biden sought to emphasize personal connections to the Jewish state and its supporters in the U.S., who responded with polite but subdued applause. He repeatedly recalled how his father, a Christian, would empathize with the plight of Israelis and their lack of control over their own security, and said Obama on his visit would make a particular point of engaging with young Israelis, I have to admit I'm a little jealous that he gets to be the one to say 'this year in Jerusalem,' but I'm the vice president. I'm not the president," he said. Follow Josh Lederman on Twitter: http://twitter.com/joshledermanAP