Vice-president tells Aipac that military option remains on the table for Obama to prevent Iran obtaining a nuclear weapon
Biden said that while the US preferred a diplomatic solution to the standoff with Iran, a military option remained on the table.
"The president of the United States cannot, and does not, bluff. President Barack Obama is not bluffing," Biden told the audience in Washington.
Israel is seeking assurances of support from the US, should it decide to launch air strikes against Iran's nuclear facilities.
There has been scepticism about Obama's commitment to a military option against Iran, given the administration's general unwillingness to be drawn into new conflicts after the experience of the Iraq and Afghanistan wars. Some observers feel that Obama's threat is aimed purely at putting pressure on Iran to resolve the standoff diplomatically and not embark on another conflict.
Pro-Israel supporters of the military option, speaking at Aipac's annual conference, attempted to counter Obama's reticence by insisting the military option would only require an overnight air strike, rather than a prolonged conflict.
In contrast with the run-up to the Iraq war, the US is determined to show that it is pursuing all alternatives to conflict. Biden said that if the US had to take military action, it was critical that the world knew it had tried everything in its power to prevent such an eventuality.
Obama, in an interview last year, insisted he was not bluffing. But his public pronouncements contrasted with behind-the-scenes US pressure on Israel to hold back from an air strike, particularly in the White House election year. The White House views Iran as the top foreign policy priority of Obama's second term.
The US and other governments held inconclusive talks with Iran recently, and further talks are planned. The new US secretary of state John Kerry, at a press conference in Saudi Arabia on Monday, said there was a "finite" time for conclusion of the talks.
Iran denies it is pursuing a nuclear weapon, and insists it only wants nuclear power to meet domestic energy needs.
Biden was speaking before a trip planned by Obama to Jerusalem later this month, provided an Israeli government is in place by then. Iran, not Palestine, is the key issue for Israel at present, though Biden stressed that the White House remained committed to a two-state solution to the Israeli-Palestinian conflict.
Obama is viewed with suspicion by many Israelis who see him as too sympathetic to the Palestinians, but Biden's credentials as a staunch supporter of Israel have seldom been challenged. Biden insisted that he and Obama were deeply committed to the security of Israel.
"It is in our naked self-interest, beyond the moral impertive," Biden said.
The vice-president said the US works hard to ensure Israel retains a qualitative edge in military hardware, even at a time of recession.
Aipac members are planning a mass lobby of Congress on Wednesday and, among other issues, will be seeking to establish whether US aid to Israel will suffer from the $85bn in budget cuts ordered by Obama on Friday.