(Reuters) - Republican U.S. presidential candidate Mitt Romney said on Thursday he did not pay less than a 13 percent tax rate at any point over the last 10 years.

Democrats have criticized Romney for not releasing more than two years of tax returns and have questioned whether the former Massachusetts governor has something to hide. Senate Majority Leader Harry Reid has accused Romney of not paying taxes for 10 years, a claim Romney denies.

"I did go back and look at my taxes, and over the last ten years, I never paid less than 13 percent," Romney told journalists in Greer, South Carolina. "I think the most recent year is 13.6 or something like that," he added.

Romney also said that when charitable donations were taken into account, his tax rate has been above 20 percent.

Romney, a multi-millionaire former private equity executive, is one of the richest men ever to run for U.S. president. He has an estimated net worth of up to $250 million.

In January, Romney released financial information showing he had paid an effective tax rate of 13.9 percent in 2010, mostly from capital gains on investments. The top tax rate for wages is 35 percent, while capital gains are taxed at a lower rate.