Donald Trump says he'll terminate Nafta

US President Donald Trump says he will shortly be providing formal notice to Congress that he will terminate the North American Free Trade Agreement (Nafta), giving lawmakers six months to approve the replacement he recently signed.

Speaking to reporters aboard Air Force One while returning to Washington from Argentina, Trump said: "I will be formally terminating Nafta shortly."

Seeking to gain leverage with sceptical lawmakers to approve the revised trade pact, Trump says Congress "will have a choice" as it considers the agreement he signed with the leaders of Mexico and Canada on Friday during the Group of 20 summit.

He says they can choose between the United States-Mexico-Canada Agreement or "pre-Nafta, which works very well".

Trump has made renegotiating NAFTA a centrepiece of his presidency.