President Donald Trump threatened to close America's southern border or large sections of it next week if Mexico does not halt illegal immigration "immediately." It is a potentially drastic step that would affect the economies of both the US and Mexico.

"It could mean all trade" with Mexico, Trump said when questioned by reporters in Florida. "We will close it for a long time. I am not kidding around."

Trump had previously threatened to close the border — he said so at a rally in Grand Rapids, Michigan, on Thursday night — but this time was different as he gave a timetable.

It's not clear whether the planned measures will also affect commercial and air travel.

"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," Trump said in a tweet. "This would be so easy for Mexico to do, but they just take our money and 'talk.'"

Trump's declaration came a day after Mexican President Andres Manuel Lopez Obrador said his country was doing its part to fight migrant smuggling.

Criminal gangs south of the border charge thousands of dollars a person to move migrants through Mexico, increasingly toward remote sections of the US-Mexico border in large groups.

"We are going to do everything we can to help," Lopez Obrador said. "We don't in any way want a confrontation with the US government."

A senior Homeland Security official on Friday suggested that Trump was referring to the ongoing surge of mostly Central American families crossing the border from Mexico.

Many people who cross the border illegally ultimately request asylum under US law, which does not require asylum-seekers to enter at an official crossing.

The official said the US might close designated ports of entry to redeploy staff to help process parents and children.

The official, who spoke to reporters on condition of anonymity, did not specify which ports the administration was considering closing, but said only that closures were "on the table."

Kevin McAleenan, the Customs and Border Protection commissioner, said Wednesday that 750 border inspectors would be reassigned to deal with the growing number of migrant families.

Democratic and Republican lawmakers have fought over whether there actually is a "crisis" at the border. The argument grew particularly heated amid Trump's push for a border wall, which he claims will solve immigration problems. However, a wall wouldn't keep out families who cross the border at official points so they can surrender and be detained.

The president, blaming Democrats, called on Congress to immediately change what he said were weak US immigration laws.

The Department of Homeland Security wants the authority to detain families for longer and more quickly deport children from Central America who arrive at the border on their own. The department argues those policy changes would stop families from trying to enter the US.

av/ng (Reuters, dpa)

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