Republican presidential nominee Donald Trump addresses the National Convention of the American Legion in Cincinnati Thomson Reuters

Donald Trump appears to have left an opening on the issue of releasing his tax returns.

In an interview with Fox News' Bill O'Reilly on Tuesday, Trump asserted that he would release his tax returns if Hillary Clinton releases emails deleted from a private server she used during her time as US secretary of state.

“When is she going to release her emails?" Trump asked. “Let her release her emails and I’ll release my tax returns immediately.”

Clinton has said that while the FBI was investigating her use of a personal email server, more than 30,000 emails that were deemed personal were deleted.

Speaking to the deleted messages, the real-estate mogul insisted Clinton "probably knows how to find them."

Trump has repeatedly declared that he will not publicly release his personal tax returns until an IRS audit into his finances is complete. It is unclear when that will happen. The GOP nominee said he has already shared nearly 100 pages of financial documents to the Federal Election Commission.

There has been no shortage of speculation surrounding Trump's reticence toward releasing his taxes.

The 2012 GOP nominee, Mitt Romney, called the move "disqualifying." Warren Buffett suggested that Trump is "afraid" to release his returns. Entrepreneur and Dallas Mavericks owner Mark Cuban implied that Trump isn't budging because the Clintons "made more" money than him.

It is customary for US presidential candidates to publicly release their taxes before the election. Every major-party candidate has done so since 1976.