President Donald Trump took a step back on Tuesday from a threat to close the U.S. southern border to fight illegal immigration, amid pressure from companies worried that such a shutdown would cause chaos to supply chains.

Trump had threatened on Friday to close the border this week unless Mexico acted. He repeated that threat on Tuesday but said he had not made a decision yet: "We're going to see what happens over the next few days."

Closing the border could disrupt millions of legal crossings and billions of dollars in trade. Auto companies have been warning the White House privately in recent days that it would lead to the idling of U.S. auto plants within days because they rely on prompt deliveries of components made in Mexico.

Trump praised efforts by Mexico to hinder illegal immigration from Central America at its own southern border on Tuesday.

"Mexico, as you know, as of yesterday has been starting to apprehend a lot of people at their southern border coming in from Honduras and Guatemala and El Salvador, and they're really apprehending thousands of people," Trump told reporters.

On Monday, the Mexican government said it would help regulate the flow of Central American migrants passing through its country. It is unclear if there has been a rise in apprehensions.

"They say they're going to stop them. Let's see. They have the power to stop them, they have the laws to stop them," Trump said.

Dozens of U.S. auto parts plants could close

Trump has made fighting illegal immigration from Mexico and Central America a key part of his agenda but shutting down one of the world's most used borders might be a step too far, even for many of his fellow Republicans.

Senate Majority Leader Mitch McConnell joined Democrats in warning Trump against such a move.

Senate Majority Leader Mitch McConnell joined Democrats in warning Trump against closing the border. (J. Scott Applewhite/Associated Press) "Closing down the border would have potentially catastrophic economic impact on our country, and I would hope we would not be doing that sort of thing," the Republican told reporters at Congress on Tuesday.

A group representing General Motors Co., Ford Motor Co. and Fiat Chrysler Automobiles NV said in a statement that "any action that stops commerce at the border would be harmful to the U.S. economy, and in particular, the auto industry."

Dozens of U.S. vehicle, engine, transmission and other auto parts plants could close because of a lack of components in the days and weeks after a border shutdown. It would also prevent thousands of vehicles built in Mexico from landing in U.S. dealer showrooms.

Automakers exported nearly 2.6 million Mexican-made vehicles to the United States in 2018, accounting for 15 per cent of all vehicles sold in the U.S. Some, like the Chevrolet Blazer SUV, are only made in Mexico.

'The system is on fire'

Senior U.S. Department of Homeland Security (DHS) officials said Tuesday a redeployment of some 750 officers on the border to deal with a surge in migrants — mostly Central American families turning themselves into border agents — had led to a slowing of legal crossings and commerce at ports of entry.

"Wait times in Brownsville [Texas] were around 180 minutes, which were two times the peaks of last year," said a senior DHS official on a call with reporters. "We ended the day yesterday at Otay Mesa [California] with a backup of 150 trucks that hadn't been processed," the official said. "This is a reality."

Migrants from Cuba rest and wait to enter the U.S. on the Mexican side of the Paso Del Norte International Bridge between El Paso and and Juarez, Mexico. (Rudy Gutierrez/The El Paso Times via AP)

DHS officials said on the call that border facilities have been overwhelmed by families who cannot be deported quickly because they hope to seek asylum in the U.S.

The U.S. Customs and Border Protection estimated that some 100,000 migrants would be apprehended or encountered at the border in March, the highest level in a decade.

"The system is on fire," a DHS official said.

Because of limits on how long children are legally allowed to be held in detention, many of the families are released to await their U.S. immigration court hearings, a process that can take years because of ballooning backlogs.

To try to address the problem, the administration in late January started returning some migrants to Mexico to wait out their U.S. court dates in Mexican border cities.

On Monday, DHS said it would dramatically ramp up the pace of that program, even as it is being challenged in court and immigration attorneys have raised concerns about how the process is being implemented.