US President Donald Trump holds piece of paper saying its his deal with Mexico as he speaks with reporters at the White House, in Washington, DC, on June 11, 2019.

President Donald Trump has said the biggest part of his new border deal with Mexico has not yet been revealed.

It may never be, according to his acting chief of staff Mick Mulvaney, who declined to discuss details of the arrangement in an interview with CNBC's Eamon Javers on Tuesday.

"If I told you, it wouldn't be the secret part of the deal, right?" Mulvaney said at the Peter G. Peterson Foundation's Fiscal Summit.

Asked when the public would see the secret deal, Mulvaney responded: "Maybe never."

"Because if it works, it doesn't make any difference," Mulvaney said. He added: "The purpose here is not to satisfy your journalistic sort of, you know, inquiries as to what the deal is. The goal is to reduce the number of people crossing the border."

Javers pressed Mulvaney on whether the U.S. had agreed to "whatever the terms are in this secret deal? We've signed up for something as a country?"

"Yeah," Mulvaney said. "Again, it's something that will kick in if the other things don't work."

In that case, Mulvaney said, the public would find out about the deal.

On Friday, the U.S. and Mexico issued a joint declaration that resolved Trump's threats to impose tariffs on Mexican imports if the country did not take action to reduce the flow of migrants across its northern border. As part of the deal, Mexico agreed to deploy its national guard to its southern border with Guatemala.

That declaration made no mention of other agreements. Mexico has flatly denied any secret deal.

But Trump has said that a secret element of the deal will soon be public.

"We have fully signed and documented another very important part of the Immigration and Security deal with Mexico, one that the U.S. has been asking about getting for many years," the president wrote in a post on Twitter on Monday. "It will be revealed in the not too distant future and will need a vote by Mexico's Legislative body!"

The secret deal has been the subject of intrigue and skepticism, particularly after Trump waved around a piece of paper on Tuesday that he said contained the agreement.

Parts of the text on the piece of paper were readable and raised the possibility that Mexico had agreed to a "safe third country" arrangement, which would require Central American migrants to request asylum in Mexico, rather than the U.S. The issue has been a sticking point in U.S.-Mexico negotiations.

Mexico's foreign minister said Tuesday that the country may consider such an arrangement if it cannot reduce unlawful immigration into the U.S. in 45 days.

The president's counselor Kellyanne Conway told MSNBC on Wednesday that she could not comment on the piece of paper Trump was holding and declined to say when the details of the secret agreement will be made public.

"I'm not going to talk about any one piece of paper," Conway said. "The president carries a lot of papers around with him at any given time."

Former U.S. diplomats have questioned the president's secret deal-making.



"There have been secret negotiations before by former presidents, but a secret deal is unprecedented," said Michael McFaul, a former U.S. ambassador to Russia and an expert on American foreign policy, in an email.

"It's also wrong," he said. "The Constitution makes clear that our founding fathers had intended the Congress to be involved in ratifying 'deals' — otherwise known as treaties — with foreign governments. Secret deals cut against the spirit of democracy."