US President Donald Trump says he is likely to shut down America's southern border next week unless Mexican authorities immediately halt all illegal immigration.
Such a severe move could hit the economies of both countries, but the president emphasised, "I am not kidding around."
"It could mean all trade" with Mexico, Trump said when questioned by reporters in Florida.
"We will close it for a long time."
Trump has been promising for more than two years to build a long, impenetrable wall along the border to stop illegal immigration, though Congress has been reluctant to provide the money he needs.
In the meantime, he has repeatedly threatened to close the border, but this time, with a new surge of migrants heading north, he gave a definite timetable.
A substantial closure could have an especially heavy impact on cross-border communities from San Diego to South Texas, as well as supermarkets that sell Mexican produce, factories that rely on imported parts, and other businesses across the US
The US and Mexico trade about $1.7 billion in goods daily, according to the US Chamber of Commerce, which said closing the border would be "an unmitigated economic debacle" that would threaten 5 million American jobs.
Trump tweeted on Friday, "If Mexico doesn't immediately stop ALL illegal immigration coming into the United States through our Southern Border, I will be CLOSING the Border, or large sections of the Border, next week."
In Florida, he didn't qualify his threat with "or large sections," stating: "There is a very good likelihood I'll be closing the border next week, and that is just fine with me."
He said several times that it would be "so easy" for Mexican authorities to stop immigrants passing through their country and trying to enter the US illegally, "but they just take our money and 'talk."'
The White House did not immediately respond to questions about whether Trump's possible action would apply to air travel.
In Florida, Trump was also asked about the two migrant children who died in US custody in December. Is the administration equipped to handle sick children who are detained?
"I think that it has been very well stated that we have done a fantastic job," he said, defending Border Patrol efforts to help the children.
Trump said that in one of the cases "the father gave the child no water for a long period of time," although an autopsy released Friday made no mention of dehydration.
"It was a very tough situation and that trek up, that's a long hard trek," he said. "It's a horrible situation. But Mexico could stop it."