Donald Trump has turned his sharp tongue on Michelle Obama, claiming the first lady once labelled his presidential rival unfit to lead the country.

Mrs Obama has proven a key ally of Hillary Clinton, vigorously campaigning for the Democratic nominee in recent weeks.

"His wife ― all she wants to do is campaign. And I see how much his wife likes Hillary," Mr Trump said of the first lady at a rally in North Carolina.

"But wasn’t she the one that originally started the statement, If you can’t take care of your home, right? You can’t take care of the White House and the country? Where is that? I don’t hear that."