President Donald Trump has issued an executive order for a wall to be built along the southern 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 that Mexico would “absolutely, 100%” reimburse the US for his wall.
Building a 2,000-mile wall along the Mexican border was one of his key pledges in the election campaign.
“We’ve been talking about this right from the beginning,” he said as he signed the actions during a ceremony at the Department of Homeland Security on Wednesday.
Mexicans see wall as racist – Will Grant, BBC News, Mexico City Mexicans might disagree about a lot – especially on the political and economic direction of the country.
But if there is one thing around which almost all of Mexico can coalesce, it is their profound opposition to the US border wall.
From the Mexican side of the border, it is seen as a policy which is intended to break up families and prevent ordinary people from looking for seasonal work in the north – the kinds of jobs, they note, which prop up the US economy.
Others go further, and consider the entire border wall to be a racist and xenophobic policy.
Either way, Mexicans from the president’s office to the factory floor agree that the country will not pay for a wall they don’t want and didn’t call for. They say they won’t finance the project either at the time of building or in the future.
That’s not to say some Mexicans aren’t in favour of change in terms of bilateral immigration.
Many are aware of the risks that their countrymen take by crossing illegally into the US, especially through dangerous border regions such as the Arizona desert.
Rather than a wall with their neighbour to the north though, they want to see comprehensive immigration reform including guest worker programmes and temporary work visas.
The executive orders are among several expected on immigration and border security this week.
They are likely to boost national security, restrict refugee access and block the issuing of visas to people from several predominantly Muslim countries in the Middle East and Africa.
Earlier on Wednesday, Mr Trump told ABC News that Mexico would pay for the wall, which he has previously estimated would cost $8bn (£6.4bn).
“We’re going to be starting those negotiations relatively soon. And we will be – in a form – reimbursed by Mexico,” Mr Trump said.
“I’m just telling you that there will be a payment. It will be in a form – perhaps a complicated form.” “It’s also going to be good for Mexico”, he added.
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