WASHINGTON, USA (AFP) — White House hopeful Mitt Romney offered his fullest explanation yet about his tax status yesterday, saying he paid at least 13 per cent a year over the past decade, but President Barack Obama’s campaign offered a prompt retort: prove it.

Romney, a multimillionaire former investor, has been under assault by Democrats for refusing to release pre-2010 tax returns, and he appeared irked by the question about his personal finances in the midst of some of the most heated rhetoric of the 2012 campaign.

“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’ve paid I find to be very small-minded compared to the broad issues that we face,” he told reporters at an airport near Greenville, South Carolina.

“But I did go back and look at my taxes, and over the past 10 years I never paid less than 13 per cent. I think the most recent year is 13.6 or something like that. So I paid taxes every single year,” he said.

Romney paid an effective tax rate of 13.9 per cent in 2010, according to returns he has released, as his income from investments was taxed as a capital gain rather than under the higher rates applied for salaried income.

Those figures may not sit well with many middle-class Americans who pay a higher rate than Romney because they pay taxes on salaried income more than investment income. The current highest US income tax rate is 35 per cent.

Obama’s campaign threw down the gauntlet, saying that “since there is substantial reason to doubt his claims, we have a simple message for him: prove it.”

“Even though he’s invested millions in foreign tax havens, offshore shell corporations, and a Swiss bank account, he’s still asking the American people to trust him,” Obama campaign spokeswoman Lis Smith said in a statement.