US President Donald Trump has said he never believed Mexico would write him a cheque for US$20 billion dollars to pay for a border wall.

"When I say Mexico is going to pay for the wall, you think they're going to write a cheque for US$20 billion or US$10 billion or US$5 billion or two cents? No, they're paying for the wall in a great trade deal," he said during his visit to the banks of the Rio Grande in the Texas border town of Mission, the Daily Mail reported.

Trump claimed that his frequent assertions that Mexico would pay for the wall were always meant to suggest an "indirect" payment.

"When during the campaign I would say Mexico is going to pay for it. Obviously I never said this and I never meant they are going to write out a cheque. I said they are going to pay for it. They are. They are paying for it with the incredible deal we made called the United States, Mexico and Canada it's a trade deal," he told reporters at the White House before he left for his visit to the US-Mexico border.

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The US-Mexico-Canada trade deal - the USMC - still has to be approved by Congress. The president complained lawmakers could hold that up because of the upcoming US elections.

"It has to be approved by Congress. Probably will be other than maybe they hold that up because they want to have - they want to do as much harm as they can. Only because of the 2020 presidential election. So, Mexico is paying for the wall indirectly and when I said Mexico will pay for the wall in front of thousands and thousands of people they are paying for the wall indirectly. Many, many times over by the really great trade deal we just made," he argued.

The president said multiple times on the campaign trail that Mexico would pay for his border wall and gave the impression America's Southern neighbour would be sending cash directly to the US for it.

'I will have Mexico pay for that wall'

During his presidential announcement speech on June 16, 2015, Trump said, "I will build a great, great wall on our southern border. And I will have Mexico pay for that wall."

And Trump's campaign posted a document detailing how Mexico will pay for the wall.

If Mexico does not pay for the wall, the administration would put that regulation into affect: "Tell Mexico that if the Mexican government will contribute the funds needed to the United States to pay for the wall, the Trump Administration will not promulgate the final rule, and the regulation will not go into effect," the document says.

The two-page proposal outlines a three-day plan that would forbid money transfer companies like Western Union from transferring funds outside the United States by anyone who couldn't prove they were in the United States legally.

The plan also included thoughts on raising trade tariffs and cancelling visas to push Mexico into coughing up the cash.

But he even joked about Mexico paying for the wall when he appeared on Saturday Night Live in 2015, pocketing a cheque for US$20 billion from its "president", played by Beck Bennett.

In recent days, Trump and his administration have been arguing Mexico will pay for the wall indirectly as they try to secure the US$5 billion in funds needed to build it.

However, one Trump adviser admitted tax payers will bear the cost.

US tax payers will pay directly for the wall, White House spokesperson Mercedes Schlapp said on CNN Wednesday.

"Yes," Schlapp said when asked about taxpayers paying for the wall. "And you know what else taxpayers are paying for? The financial burden of this illegal immigration."

The latest salvo comes on day 20 of a partial government shutdown and as both sides are at an impasse over reopening it.

Trump is holding firm for his US$5.7 billion and Democrats aren't budging from their US$1.3 billion.