President Donald Trump on Friday declared national emergency, a move that would unlock billions of dollars of federal money to construct a wall along the US-Mexico border, saying the move was essential to prevent the country from "invasion" of illegal immigrants.

Trump's move followed a rare show of bipartisanship on Thursday when legislators voted to fund large swaths of the government and avoid a repeat of the recent five-week government shutdown.

The money in the bill for border barriers, about $1.4 billion, is far below the $5.7 billion Trump insisted he needed to build the physical barrier. It would finance just a quarter of the more than 322 kilometres he wanted this year.

Amidst slamming by Democrats and human rights organisations as unlawful and abuse of his constitutional powers, Trump told reporters in the Rose Garden that the move to declare emergency was essential to prevent the US from "invasion" of illegal immigrants, drug dealers and criminal cartels.

"It's a great thing to do because we have an invasion of drugs, invasion of gangs, invasion of people," the president said.

Trump argued that he has taken this path to speed up the process of building the wall. "I could do the wall over a long period of time. I didn't need to do this, but I would rather do it much faster," he said.

Trump's move is already drawing bipartisan criticism on Capitol Hill and the opponents have already announced to legally challenge it.

However, Trump exuded confidence that he will win the court battle.

The White House said the administration has so far identified up to $8.1 billion that will be available to build the border wall once the national emergency is declared and additional funds have been reprogrammed.

This includes about $601 million from the Treasury Forfeiture Fund, up to $2.5 billion under the Department of Defence funds transferred for Support for Counter Drug Activities and up to $3.6 billion reallocated from Department of Defence military construction projects.