Threatening drastic action against Mexico, President Donald Trump declared 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 emphasized, 'I am not kidding around.'

'It could mean all trade' with Mexico, Trump said when questioned Friday 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 Friday morning, '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 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.''

Homeland Security Secretary Kirstjen Nielsen suggested Trump was referring to the ongoing surge of mostly Central American families heading north through 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.

Short of a widespread shutdown, Nielsen said the US might close designated ports of entry to re-deploy staff to help process parents and children. Ports of entry are official crossing points that are used by residents and commercial vehicles.

'If we have to close ports to take care of all of the numbers who are coming, we will do that,' Nielsen said. 'So it's on the table, but what we're doing is a very structured process based on operational needs.'

The White House did not immediately respond to questions about whether Trump's possible action would apply to air travel.

Trump's latest declaration came after Mexican President Andres Manuel Lopez Obrador said his country was doing its part to fight migrant smuggling. Criminal networks charge thousands of dollars a person to move migrants through Mexico, increasingly in large groups toward remote sections of the border.

'We want to have a good relationship with the government of the United States,' Lopez Obrador said Friday. He added: 'We are going to continue helping so that the migratory flow, those who pass through our country, do so according to the law, in an orderly way.'

Marcelo Ebrard, Mexico's foreign relations secretary, tweeted that his country 'doesn't act based on threats' and is 'the best neighbour' the US could have.

Customs and Border Protection commissioner Kevin McAleenan said Wednesday that 750 border inspectors would be reassigned to deal with the growing number of migrant families, and the DHS official said Friday that the department was seeking volunteers from other agencies to help.

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.'

Democratic and Republican lawmakers have fought over Trump's contention that there is a 'crisis' at the border, particularly amid his push for a border wall, which he claims will solve immigration problems, though a wall wouldn't keep out families who cross at official points so they can surrender and be detained.

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

Alejandra Mier y Teran, executive director of the Otay Mesa Chamber of Commerce in San Diego, said the mere threat of border closures sends the wrong message to businesses in Mexico and may eventually scare companies into turning to Asia for their supply chains.

'I think the impact would be absolutely devastating on so many fronts,' said Mier y Teran, whose members rely on the Otay Mesa crossing to bring televisions, medical devices and a wide range of products to the US. 'In terms of a long-term effect, it's basically shooting yourself in your foot. It's sending out a message to other countries that, 'Don't come because our borders may not work at any time.' That is extremely scary and dangerous.'