Mitt Romney is waiting for the Internal Revenue Service to finish processing his 2011 tax returns before releasing them, a top adviser said on Sunday.

Romney has faced mounting scrutiny regarding the contents of his tax returns, and the Obama campaign has pummeled him for not releasing more of them beyond just two years. Senior adviser Ed Gillespie deflected suggestions that the Republican presidential nominee has anything to hide, saying Romney has applied for an extension on the 2011 return in April and would provide more information by October 15, shortly before Election Day.

"Look, October 15 is the deadline for the IRS on an extension. We have said as soon as they're ready we're going to release them. And I believe they'll be ready before that," Gillespie said on "Fox News Sunday." "They're being finalized. There's a lot of forms that have to come in from other entities that the governor doesn't have control over."

Questions about Romney's tax rate have swirled around his campaign for months. Responding to intense pressure from his then-rivals for the Republican nomination, the former Massachusetts governor in January disclosed documents showing he had paid an effective rate of about 13.9 percent in 2010 -- significantly lower than the top rate. Paperwork requesting an extension on his 2011 returns estimates Romney's 2011 tax liability at around $3.2 million.

The Obama campaign has hammered Romney over his taxes, releasing advertisements highlighting the fact that the Republican nominee has stashed holdings in a Swiss bank account, as well as in Bermuda and the Cayman islands. In a Sunday appearance on "Fox News Sunday," senior Obama campaign adviser Robert Gibbs kept the heat on.

"He has clearly made a decision that what is in those tax returns is far more damaging to him than to do what every presidential candidate has done, which is show the American people your personal finances," Gibbs said.

Senate Majority Leader Harry Reid recenty provoked a firestorm after citing an employee of Bain Capital, the investment firm Romney headed, who alleged to Reid that Romney had avoided paying taxes for a decade.

The Obama campaign disavowed Reid's charges, saying the Senate's top Democrat spoke for himself, but the campaign has continued to assail Romney for not being more forthcoming. When Romney held a conference last week to counter Reid's claims, saying that he had looked over his past returns and confirmed that he had paid at least a 13 percent rate each year over the past decade, an Obama spokesperson called on Romney to provide evidence.

"Since there is substantial reason to doubt his claims, we have a simple message for him: Prove it," Obama campaign spokeswoman Lis Smith told The Wall Street Journal.