Updated 5.30pm

President Donald Trump, repeating his claim that "walls work," announced Friday that he will declare a national emergency in order to build a barrier on the US-Mexico border without funding from Congress.

"I'm going to be signing a national emergency," Trump said, repeating his claim that a wall is needed to stop an "invasion" of drugs, gangs, human traffickers and undocumented migrants over the US border with Mexico.

"Everyone knows that walls work," said Trump, whose declaration will enable him to reprogram other federal funds towards the contested wall project.

House Speaker Nancy Pelosi, Trump's Democratic nemesis in Congress, earlier warned that declaring such an emergency would be a "gross abuse of the power of the presidency."

New York state Attorney General Letitia James said Friday she would challenge the move.

"Declaring a national emergency without legitimate cause could create a Constitutional crisis," James said in a statement.

"We will not stand for this abuse of power and will fight back with every legal tool at our disposal."

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