"I think the challenge that I'll have in the debate is that the president tends to -- how shall I say it? -- say things that aren't true," Romney said in an interview with ABC News.

"It's difficult to say, well, am I going to spend my time correcting things that aren't quite accurate? Or am I going to spend my time talking about the things I want to talk about," he added, noting he was yet to choose a strategy.

With little more than 50 days to go before the November 6 election, Romney is trailing Obama in the swing states that he must win to take the White House, though the president's lead is not unassailable.

The first of three presidential debate is scheduled to take place on October 3 in Denver, Colorado.

Romney has repeatedly blamed his Democratic opponent for presiding over a feeble US economic recovery, but the attacks have failed to gain traction in opinion polls.

The Republican candidate was also forced onto the defensive after he lambasted the Obama administration's response to demonstrations that broke out at the US embassy in Cairo on Tuesday.

In a vehement attack, Romney accused the Obama administration of sympathizing with Islamist perpetrators of the attack rather than defending American values.

But the criticism was based on an embassy statement issued hours before protesters scaled the embassy wall and tore down an American flag. Administration officials insisted the embassy statement had not been cleared in Washington.

The release decried "continuing efforts by misguided individuals to hurt the religious feelings of Muslims" in a slap at a new US film deemed offensive to Islam which had prompted uproar.

Romney said he stood by his comments, which Obama said illustrated that his opponent tends to "shoot first and aim later."

"I thought the (embassy) statement was inappropriate and pointed that out," Romney said in the ABC interview.