Donald Trump, the US president, on Friday threatened to seal off the entire US-Mexico border if Congress does not approve funding for a border wall.
His comments came as the federal government shutdown entered its seventh day, with Mr Trump failing to come to a budget agreement with his Democratic opponents.
The continued standoff means around a quarter of the US government will likely remain closed well into 2019, when the Democrats will take control of the House of Representatives.
The shutdown began on Saturday after Democrats rejected Mr Trump's demand for $5 billion (£3.93 billion) for a border wall to be included in a funding bill to keep the government open.
"We will be forced to close the Southern Border entirely if the Obstructionist Democrats do not give us the money to finish the Wall & also change the ridiculous immigration laws that our Country is saddled with," Mr Trump tweeted Friday.
Analysts have warned that closing the border would cost hundreds of millions of dollars a day, with an estimated $558 billion in goods being transported across the border in both directions last year.
Such a move would also cause chaos for the nearly half a million people who are estimated to enter the US through its southern border each day.
An agreement on border funding will be the first big confrontation between Mr Trump and newly empowered Democrats, who won a majority in the House of Representatives in November's midterm elections.
The Democrats are vehemently opposed to the construction of Mr Trump's proposed wall, suggesting that funding should instead be used on border security.
As he doubled down yesterday, Mr Trump also reissued threats to shut off aid to the three Central American countries from which a majority of migrants attempting to enter the US originate.
"Honduras, Guatemala and El Salvador are doing nothing for the United States but taking our money. Word is that a new Caravan is forming in Honduras and they are doing nothing about it. We will be cutting off all aid to these 3 countries - taking advantage of US for years!" he wrote in one of a series of tweets.
The president has also signalled he is in no rush to seek a resolution, welcoming the fight as he heads toward his own bid for re-election in 2020.
Yesterday Mick Mulvaney, the president's acting chief of staff, said Mr Trump had cancelled his plans to travel to Florida for a New Year's Eve celebration. Mr Trump cancelled a planned trip last week to visit his private Mar-a-Lago club in Palm Beach, Florida, where he usually spends the Christmas period.
During an appearance on Fox News, Mr Mulvaney said that Democrats are no longer negotiating with the administration over an earlier offer to accept less than the $5 billion Mr Trump wants for the wall.
"There's not a single Democrat talking to the president of the United States about this deal," he said, admitting that the White House expected the shutdown "to go on for a while".
Nancy Pelosi, the Democratic leader in the House, has vowed to pass legislation to end the shutdown as soon as she takes control of the chamber, which is expected when the new Congress convenes on January 3.
"While we await the president's public proposal, Democrats have made it clear that, under a House Democratic Majority, we will vote swiftly to re-open government on Day One," her spokesman Drew Hammill said yesterday.
However passing any legislation will be difficult without a compromise, since the Republican-controlled Senate and Mr Trump's signature will be needed to turn any bill into law.
The shutdown is forcing hundreds of thousands of federal workers and contractors to stay home or work without pay, and many are experiencing mounting stress from the impasse.
Washington's Smithsonian museum complex announced that it would soon have to close all museums, research centres and the National Zoo in the US capital as the reserve funds it has been relying on are running dry.