President Trump on Monday repeated claims he made over the weekend, saying Mexico will ultimately pay for his proposed border wall, and that the country is "being difficult" in North American Free Trade Agreement (NAFTA) negotiations.

Trump's remarks came at a White House press conference with Finnish President Sauli Niinistö.

"One way or the other Mexico will pay for the wall," Trump said. "We're right now negotiating NAFTA. In my opinion Mexico's been very difficult, as they should be, why wouldn't they be?"

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"I guarantee you, Mr. President, Finland would never have signed NAFTA with Russia or whoever you wanted. This is not a deal that you would ever sign," Trump said.

Trump tweeted similar statements about the wall and NAFTA on Sunday, saying "we may have to terminate" the agreement.

We are in the NAFTA (worst trade deal ever made) renegotiation process with Mexico & Canada.Both being very difficult,may have to terminate? — Donald J. Trump (@realDonaldTrump) August 27, 2017

With Mexico being one of the highest crime Nations in the world, we must have THE WALL. Mexico will pay for it through reimbursement/other. — Donald J. Trump (@realDonaldTrump) August 27, 2017

Mexico responded Sunday through a press release, saying that "Mexico will not negotiate NAFTA, nor any other aspect of the bilateral relationship, through social media or any other news platform."

While the Mexican government pledged its NAFTA negotiating position will "continue to be serious and constructive," the Mexican peso fell 1.2 percent in trading Monday on news of Trump's tweets.

In its response, the Mexican government added that it would not pay for a border wall "under any circumstances" as "a principle of national sovereignty and dignity."

Still, Trump said "we may fund it through the U.S. but ultimately Mexico will be paying for it."

House appropriators added $1.6 billion for the wall in a homeland security funding bill, but it still has to go through the Senate.

The fight over wall funding promises to be a contentious issue when Congress comes back from recess in September.

Trump said last week the wall is worth a government shutdown, a claim he repeated Monday.

"If it's necessary we'll have to see, but I hope it's not necessary. The wall is needed from the standpoint of security, the wall is needed from the standpoint of drug — tremendous, the drug scourge," he said.