President Obama denied this afternoon that he is running a negative re-election campaign, noting that issues such as Republican challenger Mitt Romney's tax returns are not "in any way out of bounds."

Answering White House reporters' questions for the first time in two months, Obama contended that the tough ads being run by his campaign are fair in pointing out "sharp differences" between himself and Romney.

"I feel very comfortable with the fact that when you look at the campaign we're running, we are focused on the issues and the differences that are important to working families all across America," the president said.

"This isn't, sort of, overly personal here, guys," he added later in response to further questions about his campaign's call for Romney to release more tax returns. "This is pretty standard stuff."

Obama said his campaign had nothing to do with an ad that linked Romney to a woman's death by cancer. That ad was produced by an outside group aligned with his campaign and run by a former White House deputy press secretary.

The president also said "nobody accused Mr. Romney of being a felon," as a reporter said. Last month, Obama deputy campaign manager Stephanie Cutter said that discrepancies surrounding Romney's tenure at Bain Capital in SEC filings could constitute a "felony."

"The president falsely alleged no one in his campaign had accused Mitt Romney of committing a crime," said Romney campaign spokesman Ryan Williams. "President Obama's failure to stand up to dishonest rhetoric and attacks demonstrates yet again he's diminished the office that he holds, and his record is nothing more than business as usual in Washington."

Obama interrupted White House press secretary Jay Carney's regular briefing to take reporters' questions, noting there had been a drumbeat for him to do so in recent weeks. He had not held a press conference since June 19, when he addressed three reporters' questions at the conclusion of a G-20 economic conference.

Pressed repeatedly about the tone of his campaign, Obama defended his call on Romney to release more tax returns. He noted that the millionaire businessman has already disclosed he has "Swiss bank accounts," which are "perfectly legal" but which voters would find to be "relevant information."

Said Obama: "If you want to be president of the United States, then your life's an open book when it comes to things like your finances.

"People want to know that everybody's been playing by the same rules, including people who are seeking the highest office in the land," he said.

The policy disputes between himself and Romney include how to fix the economy, Medicare, taxes, energy and education, Obama said, adding: "That's what I talk about on the campaign."

On the other hand, he said, Romney approved an ad by his campaign that falsely accuses Obama of gutting the work requirements in the 1996 welfare overhaul.

"You can't make this stuff up," Obama said.

Also during the news conference, Obama:

Denounced the comments of Missouri GOP Senate candidate Todd Akin about "legitimate rape," calling them "offensive."

Said Obama: "Rape is rape, and the idea that we should be parsing and qualifying and slicing what types of rape we're talking about doesn't make sense to the American people -- and certainly doesn't make sense to me."

The flap shows that "we shouldn't have a bunch of politicians, a majority of whom are men, making health care decisions on behalf of women."

Asked if Republicans should pressure Akin to quit the campaign against Democratic Sen. Claire McCaskill in favor of another GOP candidate, Obama said, "I'll let them sort that out."

Again called on Syrian President Bashar Assad to step down, but acknowledged it may be difficult and bloody.

"At this point, the likelihood of a soft landing seems pretty distant," Obama said.

While he has not ordered U.S. military intervention as he did in Libya, Obama said the use of chemical or biological weapons by the Assad regime would be "a red line for us" that could "change my calculations significantly."

Said increased "green-on-blue" attacks on U.S. and NATO forces in Afghanistan by enemies dressed in green as Afghan soldiers is a result of the transition to Afghan-led security forces.

While saying the U.S. is "sticking to our transition plan," he added: "We've got to do it in a way that doesn't leave our guys deeply vulnerable."

Contributing: Richard Wolf