"If Mexico doesn't immediately stop ALL illegal immigration coming into the United States throug (sic) our Southern Border, I will be CLOSING the Border, or large sections of the Border, next week," he tweeted.
Trump, in Florida on Friday afternoon, said he could close the border to trade for a "long time" and insisted the US had run out of detention space for undocumented immigrants.
"We have the weakest, most pathetic laws," the President said, threatening to close the border if two current caravans of migrants cross into the US.
"We have run out of space. We can't hold people anywhere. Mexico can stop it so easily," he added.
Trump's comments follow a period in which the Department of Homeland Security and border officials have said their resources have become strained.
Over the last two weeks, Customs and Border Protection announced that they will be releasing migrants in Arizona, as well as the Del Rio sector and Rio Grande Valley region of Texas. And more people are expected to be apprehended at the border in March than any month since 2008, CBP says.
What does 'closing the border' mean?
So far, the plan Department of Homeland Security says it will commit to is pulling some officers from legal ports of entry to help with the influx of migrants crossing illegally. This plan will not fully close ports on the southern border as of now.
In a Friday statement announcing the move, Department of Homeland Security Secretary Kirstjen Nielsen said there is a "cascading crisis" at the border and "the system is in free fall."
The Department of Homeland Security is moving officers from its ports to help with the mission between ports, which will result in slow downs, a senior Department of Homeland Security official told reporters.
But the official said full closure of ports of entry is "on the table."
"It's short at the moment of full closure of any port, we will continue to operationally assess what is needed. But I think what the President is making clear, is with these numbers, if we have to close ports to take care of all of the numbers who are coming, we will do that, so it's on the table," the official said. "But what we are doing is a very structured process based on operational needs to determine how many additional personnel we can pull from other duties to address the crisis between the ports."
The official said the Department of Homeland Security is "looking at it from an operational perspective" and that the Department of Homeland Security secretary will make a "recommendation accordingly to the President."
Can Trump legally close the southern border?
The Justice Department declined to comment on the legality of closing the border or any portion of it, as described in the President's tweet. The Justice Department also declined to comment on whether the Office of Legal Counsel has issued an opinion.Though border closures are rare, they're not unprecedented.
For example, in 1985, the abduction of a Drug Enforcement Administration agent and subsequent threats against customs agents prompted then-President Ronald Reagan, in agreement with the Mexican government, to close nine border crossings on the southern border.