Donald Trump said the alternative to funding his controversial wall project would be total separation from Mexico.

US President Donald Trump has threatened to seal the United States-Mexico border “entirely” if Congress does not approve billions of dollars in funding for a wall.

In a burst of early morning tweets on Friday, Trump said the alternative to funding his controversial wall project would be total separation from Mexico – including making US car companies pull out their factories based on the other side of the frontier.

The threat yet again upped the ante in a political dispute that has led to a partial shutdown of the United States government and seems set to dominate the start to the third year of Trump’s presidency.

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. Hard to believe there was a Congress & President who would approve! — Donald J. Trump (@realDonaldTrump) December 28, 2018

“We will be forced to close the Southern Border entirely if the Obstructionist Democrats do not give us the money to finish the Wall,” Trump tweeted.

Trump said he would then take US-Mexican relations back to the days before the NAFTA agreement opened free trade across Canada, Mexico and the US.

That would “bring our car industry back into the United States where it belongs,” he said.

It was not clear how separating the two huge neighbours would work. Bilateral trade totalled an estimated $615.9bn in 2017, according to US government figures.

Neither did Trump make any mention of the new free trade agreement, known as the USMCA, which he only recently signed with the two neighbouring countries to replace NAFTA and which he has repeatedly praised as a huge boost for American commerce.

Democrats and some Republicans view the wall as a costly, unneeded and ineffective project, but some Republicans support the idea and back Trump’s demand for $5bn in partial funding.

Democrats, on the other hand, have refused to approve funding and Trump, who has made hard line immigration policies a centrepiece of his presidency, has retaliated by refusing to sign off on a wider spending bill, leaving some 800,000 federal employees without pay.

In Mexico, President Andres Manuel Lopez Obrador sidestepped Trump’s threat, telling journalists: “We don’t want to be imprudent and we don’t think we should get into this.”