US President Donald Trump has declared a national emergency in a bid to fund his promised wall at the US-Mexico border without congressional approval, an action Democrats have vowed to challenge.

The president’s move to circumvent Congress represents a new approach to making good on a 2016 presidential campaign pledge to halt the flow of undocumented immigrants into the country.

Trump was also expected later on Friday to sign a bipartisan government spending bill that Congress approved on Thursday.

The bill is to prevent another partial government shutdown by funding several agencies that otherwise would have closed on Saturday.

Trump had demanded that Congress provide him with $US5.7 billion ($A8b) in wall funding as part of legislation to fund the agencies.

That triggered a historic, 35-day December-January government shutdown that hurt the US economy and his opinion poll numbers.

By reorienting his quest for wall funding toward a legally uncertain strategy based on declaring a national emergency, Trump risks plunging into a lengthy legislative and legal battle with Democrats and dividing his fellow Republicans.

Fifteen Democrats in the Republican-controlled Senate introduced legislation on Thursday to prevent Trump from invoking emergency powers to transfer funds to his wall from accounts Congress has already committed to other projects.

Democratic Speaker of the House of Representatives Nancy Pelosi and the Senate’s top Democrat Chuck Schumer swiftly responded to Trump’s declaration.

“The president’s actions clearly violate the Congress’s exclusive power of the purse, which our Founders enshrined in the Constitution,” they said in a statement.

The Congress will defend our constitutional authorities in the Congress, in the courts, and in the public, using every remedy available.”

The state of New York’s attorney general Letitia James said her office would also challenge Trump in court.

“We won’t stand for this abuse of power & will fight back with every legal tool at our disposal,” James wrote on Twitter.

The president acknowledged that his order would face a lengthy court fight.

“I expect to be sued. I shouldn’t be sued … We’ll win in the Supreme Court,” Trump predicted.

Both the House and the Senate could pass a resolution terminating the emergency by majority vote.

However, that measure would then go to Trump, who would likely veto it.

Overriding the veto would require a two-thirds vote in both chambers.

Trump says a wall is needed to curb illegal immigrants and illicit drugs coming across the border.

But statistics show that illegal immigration via the border is at a 20-year low and that many drug shipments come through legal ports of entry.

When given the statistics at his Washington news conference, Trump told reporters they were “wrong”.

Also present were a half-dozen women holding poster-sized pictures of family members killed by illegal immigrants.

Trump cited their presence in announcing the emergency declaration.

Trump estimated his emergency declaration could free up as much as $US8b ($A11b) to pay for part of the wall.

Estimates of its total cost run as high as $US23b ($A32b).