Donald Trump Confirms National Emergency Over Border Wall

US President Donald Trump has announced he will declare a national emergency at the US-Mexico border, a move Democrats have vowed to challenge.

"I'm going to be signing a national emergency," Trump said from the Rose Garden of the White House on Friday.

"We have an invasion of drugs, invasion of gangs, invasion of people, and it's unacceptable," he said.

The president said he would sign the authorising paperwork later in the day in the Oval Office.

US President Donald Trump. Image: Getty

Trump was also expected on Friday to sign a bipartisan government spending bill approved by Congress on Thursday that would prevent another partial federal government shutdown by funding several agencies that otherwise would have closed on Saturday morning.

The bill, which contains no money for his wall, is a defeat for Trump in Congress.

His demand for $US5.7 billion ($A8b) in barrier funding yielded no results, other than a record-long, 35-day December-January partial government shutdown that damaged the US economy and his opinion poll numbers.

READ MORE: Trump And Democrats Fail To End Shutdown During 'Contentious' Talks 

Reorienting his wall-funding quest toward a legally uncertain strategy based on declaring a national emergency could plunge Trump into a lengthy battle with Democrats - and divide his fellow Republicans.

Fifteen Democrats in the Republican-controlled Senate introduced legislation on Thursday to prevent the transfer of funds from accounts Trump likely would target to pay for his wall.

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

House Speaker Nancy Pelosi. Image: AAP

"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 president acknowledged that his order would face a lengthy legal challenge.

"We'll win in the Supreme Court," Trump said.

Trump has argued the wall is needed to curb illegal immigrants and illicit drugs streaming across the southern border.

US President Donald Trump inspects border wall prototypes in San Diego. Image: Getty

However statistics show illegal immigration there is at a 20-year low and that many drug shipments are likely smuggled through legal ports of entry.

At Friday's White House event, a half-dozen women holding poster-sized pictures of family members killed by illegal immigrants preceded Trump into the Rose Garden. He cited their presence in announcing the emergency declaration.

A senior White House official, speaking on condition of anonymity, said the administration had found nearly $US7b ($A9.9b) to reallocate to the wall, including $US600m ($A846m) from a Treasury Department forfeiture fund, $US2.5b ($A3.5b) from a Defence Department drug interdiction fund and $US3.5b ($A4.9b) from a military construction budget.

Trump on Friday estimated the order could free up as much as $US8b ($A11b) to construct the wall.

The funds would cover just part of the estimated $US23b ($A32b) cost of the wall promised by Trump along the nearly 3200km border with Mexico.

The Senate Democrats' bill also would stop Trump from using appropriated money to acquire lands to build the wall unless specifically authorised by Congress.