JERUSALEM - Mitt Romney could not say today whether he had ever paid a tax rate lower than 13.9 percent, saying he would have to "go back and check."

In an exclusive interview with ABC News' David Muir to air tonight on World News, Romney was asked if there has ever been a year when he paid less than 13.9 percent, the rate he paid in 2010.

Transcript of Mitt Romney's Interview With ABC News

"I haven't calculated that," said Romney. "I'm happy to go back and look, but my view is I've paid all the taxes required by law."

"From time to time I've been audited as it happens, I think, to other citizens as well, and the accounting firm which prepares my taxes has done a very thorough and complete job pay taxes as legally due," said Romney. "I don't pay more than are legally due."

Pressed on whether he'd really go back and look for his tax rates as he suggested, Romney said, "I haven't looked at the tax rate paid year by year."

"I know that I pay a very substantial amount of taxes and every year since the beginning of my career so far as I can recall," said Romney.

Romney has faced mounting pressure from both Democrats and Republicans who say he should release more years of tax returns in the name of transparency.

Romney has so far released his 2010 returns and an estimate for 2011. Those two years are all Romney says he will release, arguing that no matter how many years he releases critics will always want more.