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.