Iran is still "a good way away" from acquiring a nuclear weapon, US Vice President Joe Biden said Thursday, insisting the United States would stop Tehran before it was too late.

President Barack Obama's number two came under attack during testy exchanges on Iran with Republican opposite number Paul Ryan in a one-off vice presidential election debate less than four week from election day.

"When Barack Obama was elected, they had enough nuclear material to make one bomb. Now they have enough for five. They're four years closer toward nuclear weapons capability," Ryan said.

Appearing incredulous and punctuating his reply with exclamations like "Incredible," "A bunch of stuff," and "We Irish call it malarkey," the Irish-American Biden launched a stinging counter-attack.

"Facts matter. All this loose talk about all they have to do is get to enrich uranium and they have a weapon. Not true. Not true," he said.

"If we ever have to take action, unlike when we took office, we'll have the world behind us. And that matters. That matters."

"The Israelis and the United States and the military intelligence communities are in the same exact place in terms of how close the Iranians are to getting a nuclear weapon. They are a good way away. There is no difference between our view and theirs," he said.

"These are the most crippling sanctions in the history of sanctions, period, period. Are you going to go to war? Is that what you want to do now?" the vice president challenged his opponent, 27 years his younger.

"Let's just look at this from the view of the ayatollahs," countered Ryan.

"What do they see? They see this administration trying to water down sanctions in congress for over two years. They're moving faster toward a nuclear weapon. They're spinning the centrifuges faster."

Biden, clearly irritated, replied: "Let me tell you what the ayatollah sees. He sees his economy being crippled. The ayatollah sees there are 50 percent fewer exports of oil.

"He sees the currency going into the tank. He sees the economy going into a freefall. And he sees the world for the first time totally united in opposition to him getting a nuclear weapon," he said.

"We will not allow the Iranians to get a nuclear weapon," he added.