The US President Donald Trump has confirmed that he will declare a national emergency in a bid to secure funding for his wall along the Mexican border.

Announcing the move outside the White House this afternoon, he said there are "tremendous amounts of drugs flowing into our country - much of it coming from our southern border."

Playing down the severity of the decision, he said similar declarations have been made by other presidents " many times before."

"There has rarely been a problem - they sign it but nobody cares," he said.

"They sign it for far less important things in many cases. We are talking about an invasion of our country with drugs; with human traffickers; with all types of criminals and gangs."

He claimed building the wall would save "a tremendous amount" as it would negate the need to send US troops to the border.

"We are going to be signing today and registering a national emergency and it is a great thing to do because we have an invasion of drugs; invasion of gangs; invasion of people and it is unacceptable.

"The order is signed and I will sign the final papers as soon as I get into the Oval Office.

"We will have a national emergency and we will then be sued and they will sue us in the US Circuit - even though it shouldn't be there.

"And we will possibly get a bad ruling and then we will get another bad ruling and then we will end up in the Supreme Court.

"Hopefully we will get a fair shake and then we will win in the Supreme Court."

US House Speaker Nancy Pelosi speaks after news broke of President Trump's plan to declare a national emergency, 14-02-2019. Image: Xinhua/Ting Shen

In response, US House Speaker Nancy Pelosi and Senate Democratic leader Chuck Schumer released a joint statement.

They describe the declaration as "unlawful", adding that it is "over a crisis that does not exist".

"This is plainly a power grab by a disappointed president, who has gone outside the bounds of the law to try to get what he failed to achieve in the constitutional legislative process.

"The president's actions clearly violate the Congress's exclusive power of the purse, which our founders enshrined in the constitution.

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

The statement adds: " The president's emergency declaration, if unchecked, would fundamentally alter the balance of powers".

"We call upon our Republican colleagues to join us to defend the constitution.

"Just as both parties honoured our oath to protect the American people by passing the conference committee bill, the Congress on a bipartisan basis must honour the constitution by defending our system of checks and balances.

"The president is not above the law. The Congress cannot let the president shred the constitution."

Federal shutdown

President Trump announced his intention to declare the emergency after indicating that he would sign a bi-partisan border security funding bill aimed at avoiding another Federal shutdown.

Some 800,000 federal workers were impacted by the Government shutdown – which lasted 35 days between December and January.

The shutdown began after President Trump refused to sign any funding bill that did not include $5.7bn for his border wall.

For their part, Democrats refused to provide any money for a wall they believe would be unnecessary and ineffective. They pledged extra funding for border security, just not the wall itself.

Additional reporting: Jack Quann