Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE doubled down on his promise to build a southern border wall that Mexico will pay for, even though Mexican President Enrique Peña Nieto told the Republican presidential nominee in a meeting Wednesday not to count on it.

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"Mexico will pay for the wall, 100 percent. They don't know it yet, but they're going to pay for it," Trump said during his immigration speech in Phoenix, Ariz, Wednesday night.

Peña told Trump in an unprecedented meeting between the two that Mexico would not be paying for the wall.

"At the start of the conversation with Donald Trump I made it clear that Mexico will not pay for the wall," he wrote on Twitter. "From there, the conversation addressed other subjects and developed in a respectful manner."

But during a joint press conference, Trump told reporters that they had discussed the wall, but not how it would be funded.