The acting White House chief of staff, Mick Mulvaney, on Sunday said President Donald Trump was serious in his threat to close the southern US border if Mexico did not act to curtail illegal immigration.

Trump last week threatened to order the border closed for a "long time," a move economists have warned could derail US growth prospects.

Behind the scenes, though, border and administration officials are said to believe the threatened closing wouldn't work and have reportedly said officials have not been told to prepare for a closing.

The news website Axios reported Sunday that the widespread view in both the White House and the Department of Homeland Security was that a total border closing was "a terrible and unworkable idea."

Illegal immigration has been one of the flash points of Trump's presidency, with the president elected on a pledge to stop the flow of migrants into the country.

Top White House aides on Sunday argued publicly that President Donald Trump's pledge to close the US border with Mexico was no empty threat — but border officials have raised questions anonymously over the government's power to make it happen.

Speaking on ABC News' "This Week" on Sunday, the acting White House chief of staff, Mick Mulvaney, said it would take "something dramatic" for the president to change his mind and back down from his pledge to act in a week if Mexico didn't step up. On "Fox News Sunday," the White House counselor, Kellyanne Conway, added that the president's threat was "no bluff."

Mulvaney also reiterated Trump’s pledge to cut off hundreds of millions of dollars in aid to the Central American countries of Guatemala, Honduras, and El Salvador over the issue of illegal migration.

Trump last week renewed attacks on Mexico for failing to curtail illegal immigration, threatening Friday to close the border for a "long time" if the country didn't act.

"Next step is to close the Border!" he tweeted Saturday. "This will also help us with stopping the Drug flow from Mexico!"

President Donald Trump touring the US-Mexico border at the Rio Grande. AP/Evan Vucci

But amid warnings from economists and political opponents that sealing off the border with the US's third-largest trading partner could spell mass job losses and price hikes, border and administration officials have questioned whether the order could even to put into practice.

Axios reported Sunday that the widespread view in the White House and Department of Homeland Security was that Trump's plan was "terrible" and "unworkable," while administration officials told the site it was "unlikely" Trump would follow through on his threat.

Speaking anonymously with The Washington Post, a US Customs and Border Protection official said that the Trump administration had provided no details about the president's intentions and that border officials had received no instructions to prepare to implement such an order.

The official said that to close the border, the administration would need to notify Congress and labor unions representing border officials — measures unlikely to be completed in a week, the time frame laid down by the president.

Other critics have pointed out the difficulty in keeping migrants from illegally entering the country, as they often seek out remote parts of the border to cross into US territory, where they turn themselves over to border officials and claim asylum. It can then take a long time to process asylum seekers' claims.

A US Border Patrol agent driving near the US-Mexico border fence in Santa Teresa, New Mexico in 2016. Associated Press/Russell Contreras

A senior administration official, clarifying the president's comments, told Vox on Friday that with border officials diverted from ports of entry to process unauthorized migrants, the government would be forced to close those ports of entry as a last resort.

The US Customs and Border Protection did not immediately respond to a request for comment.

Illegal immigration has been one of the flash points of the Trump presidency, with the president elected to office on the back of a pledge to build a wall along the US-Mexico border while also using inflammatory rhetoric around immigration.

Seeking to regain the initiative following the submission of the special counsel's Russia report, Trump on Friday was again talking tough on migration.

He said the US was out of detention space for unauthorized migrants and threatened to close the border if two new caravans of migrants made it into the US.