The Trump administration is looking at options for closing entry points to the US from Mexico in case the president carries out his threat to shut the border, a White House official said Wednesday.
Officials are discussing the possibility of closing some ports of entry, partly closing them, or shutting them all, the official.
US President Donald Trump appeared to retreat from the threat on Tuesday, saying that Mexico had begun to crack down on migration to the US.
But his economic advisers are looking at ways to prevent large-scale disruption to air travel and trade should he follow through.
Mexico said it had not changed its border enforcement and Republican politicians have raised alarm about the economic consequences of the US of cutting commerce with its third largest trading partner.
Mr Trump has made the threats amid a recent, sudden rise in crossings by undocumented migrants in recent weeks.
He blamed Democrats for illegal crossings, tweeting on Wednesday that if the US Congress failed to act, "the border, or large sections of the border, will close".
Mr Trump plans to highlight his policies on Friday with a trip to the border city of Calexico in California.
The US leader has struggled to make good on his signature 2016 campaign pledge to secure the southern border, and his policies have drawn criticism from both sides of Congress.
Legislators have expressed dismay that undocumented children were separated from their parents.
They refused to give the president all of the money he was seeking for a border wall, prompting Mr Trump to declare a national emergency and obtain funds from elsewhere in the budget.
Discussions about closing the border are in motion despite his comments on Tuesday that he was pleased with steps the Mexican government had taken to address migration.
"Closing down the border would have a potentially catastrophic impact,” Senate majority leader Mitch McConnell, a Republican, said on Monday. “I would hope he wouldn’t be doing that."
Mr Trump’s threats have affected markets and worried companies that have significant operations in Mexico, including car makers and agricultural businesses.
The price of Hass avocados from Michoacan, the heartland of Mexican production, jumped 34 per cent on Tuesday in the biggest one-day gain in a decade.
The US Chamber of Commerce warned on Monday that Mr Trump’s threats create "a degree of economic uncertainty that risks compromising the very gains in growth and productivity that policies of the administration have helped to achieve".
Mr Trump may feel under particular pressure to deliver substantial progress on the border after struggling to convince fellow Republicans to replace Obamacare.
On Tuesday evening he told Republican donors that the party would lose if it ran away from the issue.
But his warning came a day after he tweeted that the party would wait until after the 2020 election to vote on a replacement for Obamacare.
It seemed to be an admission that his desire to unveil a new healthcare bill, prompted by criticism of his administration backing a legal challenge that could result in millions losing coverage under the Affordable Care Act, had earned little support among congressional Republicans.
Mr McConnell said on Tuesday that he had told Mr Trump the Senate wouldn’t consider another attempt to overhaul the health-care system before the 2020 elections.
On Wednesday morning, Mr Trump tweeted that he “was never planning a vote prior to the 2020 election".
