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Mr. Biden, speaking to the American Israel Public Affairs Committee, also underscored Mr. Obama's threat to use military force — if all else failed — to stop Iran from acquiring a nuclear weapon.

"Presidents of the United States cannot and do not bluff, and President Barack Obama is not bluffing," he said to a standing ovation from about 13,000 Aipac supporters.

Moments later, Prime Minister Benjamin Netanyahu of Israel appeared by satellite from Jerusalem to say that he looked forward to thanking Mr. Obama for his support. Mr. Netanyahu said he planned to discuss with him the threats from Syria and Iran, which he said was defying diplomacy and moving closer to the "red line" with its nuclear program.

The elaborate display of harmony was a departure from some recent years, when the Aipac conference showcased tensions between the Obama administration and Mr. Netanyahu's government over issues like Jewish settlements in the West Bank or how best to curb Iran's nuclear ambitions.

There was some quiet grumbling among participants about the president's choice of former Senator Chuck Hagel as defense secretary. Mr. Hagel had been fiercely criticized by some Jewish groups for comments he made about the influence of pro-Israel lobbying groups. But Aipac did not take a public position on Mr. Hagel's candidacy.

And after Mr. Biden's speech, it issued a glowing statement, saying it welcomed "the vice president's strong statement that the president is not bluffing in his commitment to prevent Iran from obtaining a nuclear weapon."

In his last appearance before the group, soon after he took office in 2009, Mr. Biden warned that "you're not going to like this," before declaring that the Obama administration wanted Israel to stop building settlements in the West Bank, as a way to jump-start peace talks with the Palestinians.

This time, Mr. Biden did not mention settlements, and he said little about the peace process, which has been paralyzed for most of Mr. Obama's presidency despite his early efforts to play peacemaker. Administration officials have played down expectations that Mr. Obama will bring bold new proposals to revive the talks when he visits Jerusalem later this month.

Instead, Mr. Biden focused his remarks on the threats to Israel, mainly from Iran. Mr. Biden cited what he said was the president's unshakeable commitment to Israel's military superiority in the region: $3.1 billion in American military aid, including $400 million to help finance the construction of the antimissile system known as Iron Dome.

In addition to reiterating Mr. Obama's pledge, made before last year's Aipac conference, not to allow Iran to obtain a nuclear weapon, he said the United States would work with Israel to isolate terrorist groups like Hezbollah that are supported by Iran.

"Hezbollah is a terrorist organization, period," Mr. Biden said.

The vice president said that there was still a window for diplomacy with Iran, but that it was closing rapidly. He also stressed that the United States had built an unprecedented international coalition to pressure Iran, helping to halt its ascendency in the region in recent years.

Mr. Netanyahu took a darker view, saying that "diplomacy has not worked" and that Iran was "running out the clock" in nuclear negotiations with major powers. He added that sanctions had also not stopped Iran from continuing its uranium enrichment activities.

"Sanctions must be coupled with the clear and credible threat of military action," Mr. Netanyahu said.

Both Mr. Biden and Mr. Netanyahu spoke about the need to prevent the chaos in Syria from putting its stockpile of chemical weapons at risk. Mr. Netanyahu said the weapons could fall into the hands of terrorist groups like Hezbollah.

"They're like a pack of hyenas feeding on the carcass," he said, "and the carcass isn't even dead yet."

The prime minister did not appear in person at the conference, as is his custom, because he is still enmeshed in difficult negotiations to form a new government, after his Likud Party's surprisingly weak showing in elections in January.

But Mr. Netanyahu predicted that he would have a solid government in place by the time that Mr. Obama arrived, and he said that he looked forward to showing the president "a different Israel" — the high-technology powerhouse in the Middle East with a thriving economy.