U.S. President Donald Trump renewed his threat to scrap NAFTA and ripped on trading partners Canada and Mexico in a tweet early on Sunday, days before the three countries were scheduled to hold a second round of negotiations on rewriting the 23-year-old agreement.

"We are in the NAFTA (worst trade deal ever made) renegotiation process with Mexico & Canada. Both being very difficult, may have to terminate?" he wrote.

Tweet here

Trump, a Republican, promised during his campaign to overhaul or eliminate the North American Free Trade Agreement, which he cast as killing jobs and exacerbating the U.S. deficit, and to adopt a more protectionist stance for trade generally.

The first five-day round of talks between the three countries concluded last Sunday, with all sides committing to follow an accelerated process in revamping the agreement, which was originally signed by former President Bill Clinton, a Democrat whose wife, former Secretary of State Hillary Clinton, ran against Trump in the 2016 election.

Going into the five-day round of negotiations starting Sept. 1 in Mexico, Trump has kept the heat turned up. Both Mexico and Canada have dismissed his musing in a Tuesday speech that "we'll end up probably terminating NAFTA at some point" as a negotiating tactic.

(Reporting by Lisa Lambert; Editing by Andrea Ricci)