President Donald Trump on Saturday threatened to withdraw the United States from the North American Free Trade Agreement, asserting his right to broker a new trade pact that does not include Canada despite opposition from lawmakers and questions over his legal authority to do so.

Mr Trump on Friday formally informed Congress of his intent to enter into a trade deal with Mexico, with the notice adding the administration hopes Canada would be added to the new pact later.

US President Donald Trump issued a warning to both Canadian Prime Minister Justin Trudeau and Congress on Saturday in a post on Twitter. Credit:AP

US and Canadian negotiators worked throughout the week on adding Canada to Friday's notice, but the negotiations failed to produce an agreement ahead of Mr Trump's own Friday deadline.

Negotiations with Canada are set to continue Wednesday in the hopes of adding Canada to the deal, and lawmakers have told Mr Trump they would only sign onto a new NAFTA deal that includes all three North American nations.