US Vice President Joe Biden saluted his boss on Thursday as a man of steel who neutralised Osama bin Laden, as he reduced Republican White House hopeful Mitt Romney to a foreign policy neophyte "mired in a Cold War mindset."

President Barack Obama's deputy hit out at Romney for his "profound misunderstanding" of presidential responsibility, saying the challenger's go-it-alone foreign affairs platforms "would take us back to dangerous and discredited policies that would make us less safe."

Romney's campaign shot back that it was Biden who was pushing a "fantasy" foreign policy that is abdicating America's leadership role in the world, and that Obama has failed to maintain the longstanding peace-through-strength doctrine of modern US presidents.

In a New York speech one week ahead of the first anniversary of the killing of Al-Qaeda chief bin Laden by US forces in Pakistan, Biden painted Romney as weak on foreign affairs compared to Obama -- a president who he said has "a backbone like a ramrod."

"If you are looking for a bumper sticker to sum up how President Obama has handled what we inherited, it's pretty simple: Osama bin Laden is dead and General Motors is alive," Biden said, referring to the auto industry bailout.

"You have to ask yourself, if governor Romney had been president, could he have used the same slogan -- in reverse? People are going to make that judgment."

For months, the campaigns have zeroed in on the sputtering US economy, but the two sides have expanded their jabs this week to include positions on Iran, ties with Russia and Israel, and the crisis in Syria.

The attack on Romney, a former Massachusetts governor, signaled a sharpening of the Democratic Party campaign for the November 6 election, in which the president aims to win a second term.

Biden, who was Obama's point man for handling the withdrawal of US troops from Iraq, said the incumbent had kept Americans safe at home and abroad, arguing that the contrast with Romney "could not be greater."

"Al-Qaeda was resurgent and Osama bin Laden was at large. Our alliances were dangerously frayed," said Biden, referring to the legacy inherited from Republican president George W. Bush.

"President Obama ended the war in Iraq responsibly (and) set a clear strategy and end date for the war in Afghanistan," Biden said.

"Governor Romney's national security policy would return us to the past we have worked so hard to move beyond... It is no different than what governor Romney has proposed for our economy."

Biden sought to remind voters that Obama, locked in a tight race with Romney six months out from the election, "saved our economy from collapse" with some "unpopular but bold decisions," and stabilized relations with US allies.

"Governor Romney is counting on our collective amnesia, but Americans know that we cannot afford to go back to the future," Biden said, "back to a foreign policy that would have America go it alone, shout to the world you're either with us or against us, lash out first and ask the hard questions later."

Biden portrayed a candidate who at first supported the US withdrawal from Iraq but then said it was "an enormous error," who wants to keep US troops indefinitely in Afghanistan and who describes Russia as "our number one geopolitical foe."

"Governor Romney is mired in a Cold War mindset," Biden said.

And his thinking about the role of president in foreign policy "is fundamentally wrong," Biden went on. "That kind of thinking may work for a CEO, but I assure you, it will not and cannot work for a president."

Romney's camp offered sharp rebuttals in a conference call with aides and advisers.

Obama's weakness in handling crises like Syria has made the United States "a spectator on issues of national security," ex-ambassador Pierre Prosper said.

Romney foreign policy adviser Dan Senor, a former chief spokesman for the Coalition Provisional Authority in Iraq, said Obama has "downgraded" US relations with Israel and abdicated its leadership role in the past three years.

"The vice president seems to focus on a fantasy narrative, if you will, about the Obama administration's record in improving relations with the world (and) 'repairing' relations with American allies," Senor added.

On Iran, Romney adviser Alex Wong said Washington must present a credible military threat, but the Obama administration instead "has gone out of its way to convey that the military option is not serious," allowing Tehran to pursue its nuclear program without fear of retaliation.

"The foreign policy doctrine of all modern presidents has been peace through strength," Wong added. "The only two exceptions to that tradition have been Jimmy Carter and Barack Obama."