WASHINGTON — In an early-morning Twitter post, President Trump insisted on Thursday that he has been unwavering in his stance for building a border wall between Mexico and the United States to thwart illegal immigration.

He was responding to — and directly contradicting — private statements by John F. Kelly, his chief of staff, who told the Congressional Hispanic Caucus on Wednesday that Mr. Trump had not been “fully informed” when he first promised the wall, and that the president’s position on building it had “evolved” over time.

“The Wall is the Wall, it has never changed or evolved from the first day I conceived of it,” Mr. Trump wrote on Twitter at 6:15 a.m. Thursday.