WASHINGTON — US President Barack Obama warned Mitt Romney Monday he could not just "make stuff up" and defended his assault on his Republican foe's record and character in a surprise news conference.

Obama also demanded Romney release more personal tax returns, in a counter-offensive after Romney earlier bemoaned the president's tactics and accused the Democratic incumbent of not telling the truth.

His sudden appearance in the White House briefing room reflected the growing intensity of the race to November's election, as well as Romney's efforts to dent Obama's approval ratings by drawing him into a political mud fight.

Obama said his hard hitting campaign ads and rhetoric represented a fair attempt to examine Romney's credentials.

"You will see that we point out sharp differences between the candidates but we don't go out of bounds," Obama said, and then took on Romney over an attack ad on his welfare policy that fact checking groups have said is false.

"Everybody who has looked at this says that what Governor Romney is saying is absolutely wrong," Obama said, complaining that Romney was being misleading by claiming he had removed a work requirement for benefit recipients.

"The contrast, I think it is pretty stark. That is, they can run the campaign they want, but the truth of the matter is that you can't make stuff up.

"That is one thing that you learn as president of the United States, you will be called into account."

"I feel very comfortable with ... the campaign we are running."

The president also responded directly to two complaints from Romney -- over an ad by a pro-Obama group that accused him of complicity in the death of a woman from cancer and the Republican's refusal to release more tax returns.

Obama said he did not believe that the death of the wife of a steelworker, who lost his job and health insurance at a time when the firm was being run by Bain Capital, which his Republican foe founded, could be put down to Romney.

"But keep in mind, this is an ad that I didn't approve, and I did not produce, and as far as I can tell, has barely run," Obama said.

And the president demanded Romney release more than the two years of tax returns that he has already promised, noting that most Americans, unlike Romney did not have vast wealth and havens for it like Swiss bank accounts.

"The American people have assumed that if you want to be president of the United States that your life's an open book when it comes to things like your finances."

Earlier, Romney, appearing at a town hall meeting in New Hampshire with his running mate Paul Ryan, renewed his attacks on Obama's tactics -- after last week accusing the president of running a campaign of "hate."

"It seems that the first victim of an Obama campaign is the truth," Romney said. "It has been sad and disappointing."

"All we've heard so far is one attack after the other. And frankly they're typically not honest," Romney said, denying Obama's charges that he would cut taxes for the rich but raise them for the middle class.

"Mr President, stop saying something that's not the truth."