US President Barack Obama on Monday questioned whether his foe Mitt Romney would have taken out Osama bin Laden, but denied indulging in 'excessive celebration' over the terror leader's death.

Obama personally stepped into a fierce election-year row sparked by the anniversary of bin Laden's killing in a raid in Pakistan by US special forces, in a sharp, implicit criticism of his presumptive Republican White House rival.

"I said that I would go after bin Laden if we had a clear shot at him and I did it," a steely Obama said in a White House press conference, taking place on the eve of the anniversary of the US Navy SEAL raid which killed bin Laden.

"If there are others who said one thing and now suggest they would do something else, I would let them explain it," Obama said.

Obama did not mention Romney by name in his comments, which escalated a blazing campaign row over who should get credit for the daring helicopter-borne raid deep into Pakistan which killed the terrorist mastermind a year ago.

But he was clearly referring to comments Romney made in 2007, when the Republican questioned whether it would be right to infringe Pakistani territory if Islamabad refused to act on intelligence on terror suspects.

"It's wrong for a person running for the president of the United States to get on TV and say, 'We're going to go into your country unilaterally'," Romney said in August 2007.

Obama implicitly accused Romney of changing his story for political gain, in a manner which ultimately called into question the Republican candidate's leadership capacity and ability to serve as commander-in-chief.

"As far as my personal role and what other folks would do, I just recommend that everybody take a look at people's previous statements in terms of whether they thought it was appropriate to go into Pakistan and take out bin Laden," Obama said.

"I assume people meant what they said when they said it. That has been at least my practice."

Earlier, Romney was asked at an appearance in New Hampshire whether he would have given the order to eliminate bin Laden, which Obama took in launching a high-risk raid on a compound in Abbottabad, Pakistan on May 2, 2011.

"Of course, even Jimmy Carter would have given that order," Romney said referring to the 1970s Democratic president that Republicans brand as a failure on national security.

The Romney campaign hit back at Obama's comments, accusing the president of stooping to a 'cheap political ploy' and distorting Romney's 'strong policies on the war on terror'.

"President Obama's feckless foreign policy has emboldened our adversaries, weakened our allies, and threatens to break faith with our military," said Romney spokeswoman Andrea Saul.

"While the Obama administration has naively stated that 'the war on terror is over,' Governor Romney has always understood we need a comprehensive plan to deal with the myriad threats America faces."

Romney will on Tuesday mark the anniversary of bin Laden's killing by joining former New York mayor Rudy Giuliani in visiting a New York firehouse which lost several men during the September 11 attacks, campaign sources said.

Obama also responded to Republican claims that his reelection campaign has been trumpeting the killing of bin Laden a year ago in an inappropriate manner to boost his prospects of winning a second White House term in November.

"I hardly think that you've seen any excessive celebration taking place here," he said during a press conference with Japanese Prime Minister Yoshihiko Noda.

"The American people rightly remember what we as a country accomplished, in bringing to justice somebody who killed over 3,000 of our citizens," Obama said, and paid tribute to US armed forces and intelligence professionals.

The Obama campaign has clearly been seeking political gain from the raid as it tries to build a narrative that the president's qualities make him a tested commander-in-chief and that Romney is lacking in the area.

Last week, the campaign debuted an advertisement featuring former president Bill Clinton lauding Obama for taking the call to mount the raid.

Entitled One Chance, the ad used news footage including quotes from Romney during the 2008 election in which the former Massachusetts governor appeared ambivalent about going after the al-Qaeda leader.