President Donald Trump on Sunday renewed his pledge to make Mexico pay for the construction of a border wall between the U.S. and Mexico, days after threatening to trigger a government shutdown if congressional Republicans don't include funding as they tackle a spending bill due Sept. 30.

"With Mexico being one of the highest crime nations in the world, we must have THE WALL," Trump tweeted, adding that, "Mexico will pay for it through reimbursement/other."

The president did not elaborate on how Mexico would cover the cost. The White House previously has suggested that one possibility is a 20 percent tax on imports from Mexico.

In a subsequent Twitter post, Trump also said both Mexico and Canada were being "very difficult" in talks to renegotiate the North American Free Trade Agreement, which he called the "worst trade deal ever made," and said the U.S. might have to simply terminate it.

On Sunday, Mexico told Trump that it "will not negotiate" NAFTA via social media.

"Mexico will not negotiate NAFTA, nor any other aspect of bilateral relations, through social media or news media," the Mexican Foreign Ministry said in a statement.

A first round of negotiations for a new NAFTA framework ended last week.

Trump's posts about the border wall and NAFTA were part of a series of early-morning tweets.

Trump has asked for $1.6 billion to begin border wall construction, but not all congressional Republicans agree about the merits of a fight to spend potentially billions more on a border barrier as they seek to pay for tax cuts.

At a rally last week in Phoenix, Trump told supporters, "If we have to close down our government, we're building that wall," and that "one way or the other, we're going to get that wall."