WASHINGTON (REUTERS) - US President Donald Trump renewed his threat to scrap Nafta and ripped on trading partners Canada and Mexico in a tweet early on Sunday (Aug 27), 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.

In a separate Sunday morning tweet, Trump repeated his pledge that Mexico will eventually pay for his proposed border wall, saying the barrier is needed due to Mexico's high crime rate.

In response, Mexico's foreign ministry issued a statement Sunday afternoon reiterating the country's position that it will not "in any way or under any circumstance" pay for Trump's signature border wall.

The ministry added that overcoming violent crime associated with cross-border drug trafficking is the responsibility of both nations, pointing to the high demand for drugs in the United States from Mexico and other countries.

Trump, a Republican, promised during his campaign to build the wall and overhaul or eliminate Nafta (the North American Free Trade Agreement), which he cast as killing jobs and exacerbating the US 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.

The second round of Nafta talks will kick off on Friday in Mexico City.

Mexico's negotiating position will continue to be "serious and constructive" and the country's negotiators will not hash out differences "via social media or the press," the foreign ministry's statement said.

Going into the next round of Nafta talks, 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.