Prototypes for U.S. President Donald Trump's border wall with Mexico are shown near completion in this picture taken from the Mexican side of the border, in Tijuana, Mexico, October 23, 2017.

President Donald Trump on Thursday, in an apparent challenge to his chief of staff, defended his proposed wall along the U.S. southern border, saying his position "has never changed or evolved," and that Mexico would pay for the barrier "directly, or indirectly."

"The Wall is the Wall, it has never changed or evolved from the first day I conceived of it," Trump wrote in a pair of early morning tweets. "The $20 billion dollar Wall is 'peanuts' compared to what Mexico makes from the U.S. NAFTA is a bad joke!"

Trump tweet 1

Trump tweet 2

Trump's tweets came hours after White House chief of staff John Kelly told Fox News in an interview that the president had evolved on several issues, including immigration.

Kelly, in his interview with Fox News, also did not deny a Washington Post report that he told Democratic lawmakers that some of the president's tough immigration views during the 2016 president campaign were "uninformed," particularly when it came to the border wall that was at the center of Trump's pitch to voters.

There will not be "a concrete wall from sea to shining sea," Kelly reportedly told attendees of his meeting Wednesday, according to the Post. Instead, Kelly told lawmakers, "concrete wall would be good in only certain places."

-CNBC's Mike Calia contributed to this article.