By Yeganeh Torbati and Jeff Mason

WASHINGTON/LAKE OKEECHOBEE, Fla. (Reuters) – President Donald Trump threatened on Friday to close the U.S. border with Mexico next week, potentially disrupting millions of legal border crossings and billions of dollars in trade if Mexico does not stop immigrants from reaching the United States.

“We’ll keep it closed for a long time. I’m not playing games. Mexico has to stop it,” Trump said on a visit to Florida. Asked if he would close the Mexican border to all trade, Trump told reporters: “It could be to all trade.”

Trump has repeatedly vowed to close the U.S. border with Mexico during his two years in office and has not followed through. But this time the government is struggling to deal with a surge of asylum seekers from countries in Central America who travel through Mexico.

Department of Homeland Security officials warned that traffic with Mexico could slow as the agency shifts 750 border personnel from ports of entry to help process asylum seekers who are turning up between official crossing points.

“Make no mistake: Americans may feel effects from this emergency,” Homeland Security Secretary Kirstjen Nielsen said in a statement. Nielsen said the personnel shift would lead to commercial delays and longer wait times at crossing points.

Nielsen and other U.S. officials say border patrol officers have been overwhelmed by a dramatic increase in asylum seekers, many of them children and families, fleeing violence and economic hardship in El Salvador, Honduras and Guatemala.

March is on track for 100,000 border apprehensions, the highest monthly number in more than a decade, U.S. Customs and Border Protection Commissioner Kevin McAleenan said on Wednesday. Some 90,000 will be able to remain in the United States while their asylum claims are processed, he said.

Mexico played down the possibility of a border shutdown.

“Mexico does not act on the basis of threats. We are a great neighbor,” Mexican Foreign Minister Marcelo Ebrard said on Twitter.

It is not clear how shutting down ports of entry would deter asylum seekers, as they are legally able to request help as soon as they set foot on U.S. soil.

But a border shutdown would disrupt tourism and commerce between the United States and its third-largest trade partner, which totaled $612 billion last year, according to the U.S. Census Bureau.

“We’d be looking at losses worth billions of dollars,” said Kurt Honold, head of CCE, a business group in Tijuana, Mexico, in response to Trump’s threat. “It’s obvious he’s not measuring what he says.”

U.S. ports of entry recorded 193 million pedestrian and vehicle-passenger crossings last year, according to the U.S. Department of Transportation.

As president, Trump has legal authority to close particular ports of entry, but he could be open to a legal challenge if he decided to close all of them immediately, said Stephen Legomsky, a former chief counsel at U.S. Citizenship and Immigration Services under Democratic President Barack Obama.

Trump is trying to convince Congress to sign off on a revised trade agreement with Mexico and Canada that his administration negotiated last year.

“Mexico has for many years made a fortune off of the U.S., far greater than Border Costs. 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,” he said on Twitter.

Trump launched his presidential bid in June 2015 with a promise to crack down on illegal immigration, saying Mexico was sending rapists and drug runners into the United States.

Mexican President Andres Manuel Lopez Obrador said on Thursday that tackling illegal immigration is an issue chiefly for the United States and the Central American countries to address.

Trump has so far been unable to convince Congress to tighten asylum laws or fund a proposed border wall, one of his signature policies. Trump has declared a national emergency to justify redirecting money earmarked for the military to pay for its construction.

(Reporting by Yeganeh Torbati and Jeff Mason; Additional reporting by David Alexander, Anthony Esposito, Lizbeth Diaz and Andy Sullivan; Writing by Andy Sullivan; Editing by Tim Ahmann and Grant McCool)