Trump: Mexico will pay for 'great wall' at US border -- 'but they don't know it yet'
Donald Trump speaks at a campaign rally in Phoenix in Aug. 31, 2016. (YouTube)

Donald Trump rebounded from his meeting in Mexico with that country's president to reiterate his campaign promise of a wall along the Mexico-US border.

"We will build a great wall," Trump said at a rally in Phoenix on Wednesday night, before adding that Mexico will pay for it, only "they don't know it yet."

Trump's boisterous tone at the event marked a contrast from his more subdued appearance alongside Mexican President Enrique Peña Nieto earlier in the day, at which time he said that payment for the hypothetical wall had not been discussed.

That statement was immediately contradicted by Peña Nieto's administration, which said in a statement that he told Trump that Mexico would not foot the bill for his idea.