US President Donald Trump insisted Thursday that his plan for a wall along the Mexican border has "never changed or evolved," in tweets posted after his chief of staff said he was not "fully informed" when he pledged to build it last year.

Retired General John Kelly's remarks, made to members of the Congressional Hispanic Caucus and reported by the New York Times Wednesday, were a rare departure from the president on one of the core issues that defined his upstart run for office.

He told the lawmakers he had persuaded Trump the wall was not necessary and that the president's opinion on the barrier had "evolved."

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But Trump hit back on Twitter, writing: "The Wall is the Wall, it has never changed or evolved from the first day I conceived of it."

The president added that some of the wall will be "see through" -- a protection, he said last July, against people throwing "large sacks of drugs" over -- and repeated that it will be paid for "directly or indirectly" by Mexico.

"The $20 billion dollar Wall is 'peanuts' compared to what Mexico makes from the U.S. NAFTA is a bad joke!" he said, reasserting his position on the trade pact which is currently being renegotiated.

Meanwhile, Trump also clarified the wall would not be built in areas of natural protection.

The Twitter burst comes amid rocky efforts in Congress to reach a deal on funding to avert a government shutdown, which could come as early as midnight Friday unless a short term fix is found.

Democrats have demanded a broader deal that includes continued protection against deportation for hundreds of thousands of immigrants who arrived in the country as children.

"If there is no Wall, there is no Deal!" Trump said in another tweet that described Mexico as "now rated the most dangerous country in the world."

Kelly was brought in as chief of staff six months ago in a bid to put order to the command center of Trump's chaotic presidency.