Joe Biden rained rhetorical punches on Republican Paul Ryan on Thursday in a fiery vice-presidential debate, desperate to stem Democratic Party panic after US President Barack Obama‘s lifeless display last week.

US Vice-President Joe Biden (second left), Republican vice-presidential candidate Paul Ryan and their families speak after the vice-presidential debate at Centre College on Thursday in Danville, Kentucky. Kevin Lamarque / Reuters

Vice-President Biden and Ryan clashed sharply on Iran, Libya, Afghanistan and top domestic issues including healthcare, abortion and taxes on Thursday in Kentucky.

Biden also criticized Republican nominee Mitt Romney over his remark to rich donors that 47 percent of US citizens were "victims" reliant on government, an attack Obama avoided in his debate loss to Romney, to the fury of Democrats.

In a crackling showdown, Biden, 69, came under intense pressure from Ryan on the Obama administration‘s handling of the crisis sparked by the killing of Chris Stevens, the US ambassador to Libya, in Benghazi on Sept 11.

"We will find and bring to justice the men who did this. Whatever mistakes were made, will not be made again," a steely Biden vowed, and then swiftly went on offense, touting Obama‘s record on national security.

Passionate and adamant, Biden was on a mission to reverse Romney‘s polling surge and steady worried Democrats, and drew on his years as a foreign policy expert to repel his well-briefed opponent‘s attacks.

But his constant quirky smirking and guffaws on a split screen when the earnest Ryan, 42, gave his answers risked distracting millions of television viewers watching at home, and could dominate the post-debate fallout.

Biden highlighted Obama‘s honored promise to end the war in Iraq, his effort to bring troops home from Afghanistan by 2014 and his decision to make the operation to hunt and kill Osama bin Laden a top priority.

"The president of the United States has led with a steady hand and clear vision. Governor Romney, the opposite," he charged.

Ryan, a Wisconsin congressman, replied with a fierce assault on Obama‘s foreign policy and complained that the US ambassador in Paris had a Marines detachment while Stevens, in restive Libya, did not.

"Look, if we‘re hit by terrorists, we‘re going to call it for what it is, a terrorist attack," Ryan said, hitting a Republican theme that Obama did not want to admit the truth for political reasons.

Ryan also charged there was a deeper problem for Obama.

"What we‘re watching is the unraveling of the Obama foreign policy, which is making things more chaotic and us less safe," Ryan said.

Biden replied: "With all due respect, that‘s a bunch of malarkey."

Immediate analysis by pundits scored the clash a draw, with Biden apparently doing enough to cheer gloomy Democrats and Ryan avoiding serious errors that could haunt the Republican ticket.

Obama, who watched the debate aboard Air Force One, told reporters he "could not be prouder" of Biden and his strong advocacy of the middle class.

Romney called Ryan to congratulate him on his performance, aides said.

With Iran‘s nuclear drive casting a shadow over US immediate foreign policy, Biden said that the Islamic republic was still a "good way away" from getting an atomic bomb and vowed it would never get there.

He accused Romney and Ryan of recklessly talking tough on Iran.

"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.

Ryan, however, claimed Obama had been asleep at the switch.

"Let‘s just look at this from the view of the ayatollahs," he said.

"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 also took on Romney over his remark in a secretly filmed fundraiser with rich donors that 47 percent of Americans are "victims" who dodge taxes, rely on the government and will not take responsibility for themselves.

"These people are my Mom and Dad, the people I grew up with, my neighbors. They pay more effective tax than Governor Romney pays in his federal income tax," Biden said, making an attack Obama eschewed last week.