President Donald Trump has issued an executive order for an “impassable physical barrier” to be built along the US border with Mexico.
He also signed an action to strip funds from US cities that are sanctuaries for undocumented immigrants.
Mr Trump said in a TV interview with ABC News that Mexico would “absolutely, 100%” reimburse the US for his wall.
But Congress would have to approve funding for the structure, which is estimated to cost billions of dollars.
Building a 2,000-mile barrier along the Mexican border was one of Mr Trump’s key pledges in the election campaign.
He spoke of a “crisis” on the southern US border as he signed the directives during a ceremony at the Department of Homeland Security on Wednesday.
The orders also called for hiring 10,000 immigration officials to help boost border patrol efforts.
“A nation without borders is not a nation,” he said. “Beginning today the United States gets back control of its borders.”