U.S. Republican presidential candidate Newt Gingrich released his 2010 tax returns Thursday night as the final debate before the South Carolina primary got underway.

The tax returns showed Gingrich and his wife owed federal taxes of 994,708 U.S. dollars , or 31 percent of their income, on an adjusted gross income of over 3 million dollars.

The Gingrich campaign said 613,517 dollars of the tax amount owed had been previously withheld or otherwise paid, and the couple paid the remaining balance of 382,734 with their filing.

The filing showed the couple's wage and salary income was about 450,000 dollars. They also earned over 40,000 dollars from speaking and board of directors fees.

The biggest portion of their income, over 2.5 million, is from partnerships and S corporations, a type of corporation that does not pay federal income taxes, but divides income or losses among shareholders, who in turn report the income or loss on their own individual income tax returns.

Gingrich's tax returns release comes as the remaining four GOP candidates are facing off in their last debate before the South Carolina primary, the first one in the South, gets underway on Saturday.

Former Massachusetts governor Mitt Romney, the billionaire frontrunner in the race, is facing increasing pressure to release his own tax returns.

Romney, when pressed on the issue during the debate in Charleston, South Carolina, said he would release his tax returns if he becomes the Republican nominee.

VietNamNet/Xinhuanet