President Donald Trump yesterday threatened to seal off the US-Mexico border "entirely" if Congress does not approve billions of dollars in funding for a wall.

In a burst of early morning tweets the president said the alternative to funding his hugely 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 row that has led to a partial shutdown of the US 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," Trump tweeted.

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

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

Trump wants $5 billion in funding for a wall along the more than 2,000-mile border, which he says is currently too porous to stop illegal immigration and which he says has become a magnet for criminals, drugs and even terrorists.

Opponents -- especially in the Democratic party but also some in Trump's Republican party -- say that a physical wall is impractical and that the idea is being used as a political tool to whip up xenophobia in Trump's right-wing voter base.

Both sides have dug in. Democrats refuse to approve funding and the president -- who has made hardline immigration polices a centerpiece of his presidency -- has retaliated by refusing to sign off on a wider spending bill, leaving some 800,000 federal employees without pay.