WASHINGTON -- Mitt Romney said Thursday that over the past 10 years, he had never paid less than a 13 percent rate in income taxes, but he refused again to release more of his returns.

Soon to be nominated as the Republican presidential standard-bearer, Romney said he had gone back and looked at his taxes, and found that "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."

Earlier this year, Romney released his 2010 returns and estimates of his 2011 taxes. In 2010, he paid taxes at a 13.9 percent rate. For 2011, he expected to pay a 15.4 percent rate.

At a news conference Thursday in Greer, S.C., Romney tried to defuse the political controversy over his stance, saying the commotion was "small-minded."

And he took aim at Senate Majority Leader Harry Reid, D-Nev., who charged earlier this month on the Senate floor that Romney had paid no taxes for a decade, but offered no proof. Reid said he had heard that from a Bain Capital investor whom he refused to identify. Romney co-founded Bain, a private equity firm.

"I paid taxes every single year," Romney said. "Harry Reid's charge is totally false. I'm sure waiting for Harry to put up who it was that told him what he says they told him."

On Thursday, Reid wouldn't back down.

"We'll believe it when we see it," said his spokesman, Adam Jentleson. "Until Mitt Romney releases his tax returns, Americans will continue to wonder what he's hiding. Romney seems to think he plays by a different set of rules than every other presidential candidate for the last 30 years, all of whom lived up to the standard of transparency set by Mitt Romney's father and released their tax returns."

When Romney's father, George Romney, was the governor of Michigan and ran for president in 1968, he released 12 years' worth of returns.

Reid has refused to release his own tax returns, despite repeated requests from McClatchy Newspapers.

Romney's comments are the latest chapter in a saga that has kept his presidential campaign on the defensive. President Barack Obama's forces have relentlessly demanded release of the returns, suggesting that Romney is hiding something.

"He has the ability to prove his claim," Obama campaign spokesman Ben LaBolt said. "The American people deserve the opportunity to look through these documents and make their own conclusions. ... We would say, prove it, Mr. Romney."

Romney didn't explain Thursday why he won't release the returns. His comments came at the end of a news conference where other topics were discussed.

"I just have to say, given the challenges that America faces -- 23 million people out of work, Iran about to become nuclear, one out of six Americans in poverty -- the fascination with taxes I paid, I find to be very small-minded compared to the broad issues we now face," he said.

In January, Romney reported owing $6.2 million in federal taxes on $42.5 million in income over the past two years. His campaign released more than 500 pages of documents.

The data showed that Romney had offshore accounts in the Cayman Islands and his blind trust maintained a Swiss bank account until 2010.

It revealed that the Romneys had an adjusted gross income of $21.6 million that year and paid about $3 million in taxes. The couple also donated $2.9 million to charitable causes.

Since virtually all their income was from past investments and taxed as capital gains, they had an effective 2010 rate of taxation of 13.9 percent. Last year, Romney earned an estimated $20.9 million, and he expected to pay $3.2 million, for a 15.4 percent rate. Ann Romney, the candidate's wife, has defended the decision not to release any more returns.

"We have been very transparent to what's legally required of us," she said in an interview to be broadcast Thursday on NBC. "There's going to be no more tax releases given."

If they release any more information, she said, "it will only give them more ammunition," referring to her husband's political opponents. "There's nothing we're hiding."