U.S. investor Warren Buffett, speaking at a campaign rally for Democratic presidential candidate Hillary Clinton, challenged Republican Donald Trump on Monday to release his tax returns.

Trump has said he would not release his returns until the Internal Revenue Service has completed an audit.

"Now I've got news for him, I'm under audit too and I would be delighted to meet him anyplace, anytime, before the election," said Buffett, the chief executive officer of Berkshire Hathaway. "I'll bring my tax return, he can bring his tax return ... and let people ask us questions about the items that are on there."