President Barack Obama on Tuesday slapped down Donald Trump for seeking to discredit next month's election with allegations of vote rigging, telling the Republican nominee to "stop whining."

Facing sinking poll numbers and accusations of sexual assault, Trump has stepped up charges that the White House race is being rigged against him, warning of "large scale voter fraud."

"There's no evidence that that has happened in the past, or that there are instances in which that will happen this time," Obama told a news conference.

"And so I'd advise Mr Trump to stop whining and go try to make his case to get votes."