Washington (CNN) President Donald Trump on Friday repeated his threat to close down the border between the United States and Mexico, but this time he noted he would act next week if Mexico doesn't step up.

"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.

Earlier this week, Customs and Border Protection Commissioner Kevin McAleenan said the "breaking point has arrived" for the US immigration system.

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?

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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.

Will closing the border stem the flow of migrants illegally crossing into the US?

The Department of Homeland Security official said collaboration with Mexico and shifting resources might start to stem the flow.

However, part of the crisis at hand appears to be the number of individuals and families seeking asylum in the US. Often times, asylum seekers will cross illegally into the US awaiting to be found and processed by a border agent. More personnel between ports of entry may allow more visibility to catch these migrants once they've crossed, but it's unclear whether their presence will make them more likely to avoid crossing in the first place.

It's also unclear whether the closure be limited to individuals crossing into and out of the US. If it includes cross-national trains and trucks importing and exporting goods with Latin America, a closure could pose a major threat to international commerce.

Robert Perez, deputy commissioner for Customs and Border Protection, told "CNN's Right Now with Brianna Keilar" on Friday that the closure of ports of entry would have "pretty severe" consequences.

"It's Customs and Border Protection at every port of entry. Nearly 400 million travelers a year, $2.3 billion worth of trade, nearly 30 million trucks, rail cars and cargo containers every year," Perez said. "And so, there (will be) a severe impact and this is important to understand because ... what is so important to understand is that the crisis is so severe right now, we are literally trying to save lives."

Will Trump make good on his threat?

The President seems to be hinting at news to come during a border press event in the coming weeks, at least that's what he told Fox News host Sean Hannity earlier this week.

"In two weeks, I'm going to a location, it's going to be in California. Believe it or not, they were begging us to build in San Diego, begging us to build a wall. And we're just in the process of completing a major stretch of many miles of wall. And we're going to have a news conference there just to show people that ... it's really good stuff," Trump said.

He also teased the visit at a rally in Michigan on Thursday.

But Trump has threatened to close the southern border prior to Friday's ultimatum, and he hasn't exactly followed through with it.

Trump tweeted in December that the US will be "forced to close the Southern Border entirely if the Obstructionist Democrats do not give us the money to finish the Wall & also change the ridiculous immigration laws that our Country is saddled with." But he didn't close the border then.

And last November, a major US-Mexico border crossing in San Diego was closed for hours after a group of hundreds of migrants on the Mexican side rushed the border area, leading US Border Patrol agents to fire tear gas at the group.

The President said at the time that "if we find that it's uncontrollable" or it gets to a point where "our people are going to start getting hurt, we will close entry into the country for a period of time until we can get it under control."

"The whole border. I mean the whole border," Trump said.

But again, he didn't make good on the threat at the time.