US Presidential Debate 2016: Trump promises to release tax returns, but only if Clinton releases 33,000 deleted emails

Hillary Clinton and Donald Trump have traded blows on the debate stage over her use of a private email server as secretary of state, and the Republican billionaire's refusal to release his tax history.

"I will release my tax returns against my lawyer's wishes when she releases her 33,000 e-mails that have been deleted," Mr Trump said when pressed on the issue during their first White House debate today.

READ MORE: US Presidential Debate: Trump and Clinton

"I have no reason to believe that he's ever going to release his tax returns, because there's something he's hiding," Ms Clinton fired back.

"The only years that anybody's ever seen were a couple of years when he had to turn them over to state authorities when he was trying to get a casino license, and they showed he didn't pay any federal income tax," she added.

As Ms Clinton criticised Mr Trump about the tax issue, he interjected to say "that makes me smart".

Ms Clinton later countered by suggesting Mr Trump's not paying taxes had contributed to a lack of government funding.