The Obama administration has helped change global opinion on Iran, US Vice President Joe Biden said Tuesday, pledging that Washington would use "whatever means" to keep Tehran from acquiring a nuclear weapon.

"When we took office, let me remind you, there was virtually no international pressure on Iran. We were the problem," said Biden, speaking to a group of conservative rabbis in Atlanta.

"We were diplomatically isolated in the world, in the region, in Europe. The international pressure on Iran was stuck in neutral," Biden said, criticizing the policies of the administration of former president George W. Bush.

"We were being criticized in European capitals for being unilateral. And Tehran had allies; they were intimidating their neighbors," he said. "America's leadership was in doubt.

"We were neither fully respected by our friends nor feared by our opponents. Today it is starkly, starkly different," Biden said, touting President Barack Obama's foreign policy acumen as he seeks re-election in November.

Pointing out that a "greatly diminished" Syria was Iran's only ally, Biden said: "Not just because of the legitimate threat but because of the president's efforts, Iran is now isolated and the United States is not isolated."

The West and Israel suspects Iran is trying to develop a nuclear weapon under cover of a civilian energy program, but Tehran insists the program is solely peaceful.

Biden said Israel's view that a nuclear-armed Iran poses an existential threat to the Jewish state was "justifiable," and added: "An Iran with nuclear weapons would also pose a great threat to US security."

"The United States policy under President Barack Obama is not one of containment," said Biden, who served for more than 30 years in the Senate and was the head of the Senate Foreign Relations Committee.

"It is straightforward: We will prevent Iran from acquiring a nuclear weapon by whatever means we need."