"The wall is getting designed right now. A lot of people say, 'Oh, oh, Trump was only kidding with the wall.' I wasn't kidding. I don't kid. I don't kid," Mr Trump told conference of County Sheriffs at the White House.
The wall, a major poll promise, is essential to stop the flow of illegal immigrants and drug pouring into the country from across the border, he said.
"I watch this, and they say I was kidding. Nah, I don't kid. I don't kid about things like that, I can tell you. No, we will have a wall. It will be a great wall and it will do a lot of -- it'll be a big help," he said.
Mexican President Enrique Pena Nieto and US President Donald Trump spoke by phone on January 27, a Mexican official said, amid diplomatic tensions over Trump's border wall plan.
On January 25, Trump signed an executive order telling officials to begin to "plan, design and construct a physical wall" along the 3,200-kilometre (2,000-mile) US-Mexico border, making good on a campaign pledge.
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