Mitt Romney has paid no less than 13 percent in personal income taxes over the past 10 years, he said Thursday.

The presumptive GOP nominee has faced withering criticism from Democrats over the release of his tax returns, including a charge by Senate Democratic Leader Harry Reid that Romney had paid no taxes for a 10-year period. Reid did not specify if those were the last 10 years, or an earlier period.

He expressed frustration with Reid's attack at a press conference with reporters in South Carolina.

"I did go back and look at my taxes and over the past 10 years I never paid less than 13 percent. I think the most recent year is 13.6 or something like that. I paid taxes every single year," Romney said, explaining he had gone back to check his records after being asked by a reporter about the tax rates he had paid. "Harry Reid's charge is totally false. I am sure waiting for Harry to put up who it was that told him what he says they told him - I don't believe it for a minute by the way - but every year I paid at least 13 percent and if you add in addition the amount that goes to charity, why, the number gets well above 20 percent."

Reid had told reporters an anonymous source gave him a tip. A source close to Reid later said that Reid got the information from an investor with Bain, the private equity firm Romney ran.

Romney has released his 2010 tax returns and an estimate of his 2011 tax returns when he filed for an extension, but has declined to make available any earlier returns.

On Thursday he called insistent questions over his tax documents "small-minded compared to the broad issues that we face" such as unemployment and the nuclear threat from Iran.