U.S. Republican presidential candidate Mitt Romney is under increasing pressure Tuesday to release tax returns, as rivals and the White House blast him for not doing so.

File photo of U.S. Republican presidential hopeful Mitt Romney [Xinhua]

Romney, a billionaire, on Tuesday told a campaign event in South Carolina that he pays a tax rate of about 15 percent because so much of his fortune comes from past investments, and he may release his tax returns in April.

White House Spokesman Jay Carney said the figure reveals an unfairness in the tax code that President Barack Obama is concerned about.

"This only illuminates what he believes is an issue, which is that everybody who's working hard ought to pay their fair share," said Carney. "That includes millionaires who might be paying an effective tax rate of 15 percent when folks making 50,000 dollars or 75,000 dollars or 100,000 dollars a year are paying much more."

Romney's opponents in the GOP field has urged the former Massachusetts governor to release his tax returns. Rick Perry, the Texas governor who is trailing in polls, and Newt Gingrich, the former House speaker, both attacked Romney on the issue during a debate Monday evening.

Gingrich, for his part, said Romney's suggestion to consider releasing his tax returns in April was "nonsensical," because by then he might already have become the de facto Republican nominee.

"What is he saying to the people of South Carolina? You're not important enough for me to release my income tax, nor the people of Florida?" said Gingrich. "Either there is nothing there, so why isn't he releasing it, or there is something there so why he is hiding it?"