Update 3:50pm EST:

White House Press Secretary Sean Spicer indicated that Trump plans to pay for the border wall by imposing a 20 percent tariff on all imported goods, provoking fears of a trade war:

NEWS: @PressSec tells pool that Trump intends to pay for wall by imposing a 20 percent tax on all imports from Mexico. — Zeke Miller (@ZekeJMiller) January 26, 2017

Let Trump's trade war with Mexico begin! https://t.co/jkn5uazgsx — Jordan (@JordanChariton) January 26, 2017

Update 12pm EST:

Mexico President Enrique Peña Nieto has canceled a planned meeting with President Donald Trump—which would have been the first between the two leaders—apparently in response to Trump's hostile comments about Mexico's refusal to pay for a new U.S.-Mexico border wall.

Peña Nieto announced his decision on Twitter. "This morning we have informed the White House that I will not attend the meeting scheduled for next Tuesday with the @POTUS," the tweet says, according to an online translation.

Amid the dust-up about the border wall, it emerged Thursday that U.S. Border Patrol chief Mark Morgan is leaving the agency, the Associated Press reported, although it is unclear whether he resigned or was asked to leave.

As many have observed, Trump's insistence on a new border wall between the U.S. and Mexico comes as undocumented border crossings have already sunk to a 40-year low.

Earlier:

Mexico President Enrique Peña Nieto on Wednesday once again asserted that Mexicans will not pay for President Donald Trump's border wall between the U.S. and Mexico—and Trump responded with a series of aggressive tweets challenging Nieto to cancel an upcoming meeting between the two leaders.

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"If Mexico is unwilling to pay for the badly needed wall, then it would be better to cancel the upcoming meeting," Trump tweeted early Thursday.

Mexican leaders have already been urging Nieto to cancel the meeting in response to Trump's hostility, NPR reported Wednesday.

Trump on Wednesday issued an executive order for the construction of the wall.

It remains unclear whether other Republican politicians believe Mexico will in fact finance the wall. U.S. House Speaker Paul Ryan (R-Wis.) said that the U.S. would pay for the wall in an interview on MSNBC Wednesday. "We're going to pay for it," he admitted.

Ryan did, however, make the vague comment that there were "various ways" to get Mexico to finance it, implying that Mexicans would reimburse the U.S. for the cost of the wall. Trump also insisted that Mexico will fund the wall in an ABC interview on Wednesday. But Mexico has repeatedly asserted that it will not, in any scenario, pay for Trump's border wall.

The border wall with Mexico—and the promise that "Mexico will pay for it"—was one of the pillars of Trump's presidential campaign.

In his comments Wednesday, Peña Nieto said: "I regret and condemn the United States's decison to pursue the construction of a wall that far from uniting us, has been dividing us for years. Mexico does not believe in walls. I have said it over and over again: Mexico will not pay for any wall."

Watch Nieto's announcement, dubbed in English, here:

A Spanish version is available here.

U.S. Senate Majority Leader Mitch McConnell (R-Ky.) said Thursday that the wall is estimated to cost $12 to $15 billion. He made those comments outside of the GOP retreat in Philadelphia in a brief press conference with Ryan, during which McConnell and Ryan refused to say exactly how the wall would be financed, or to confirm whether or not Mexico would pay for it. Ryan added that he expects Trump's White House to submit a plan for the wall's financing in coming days.