The US acting secretary of defence has authorised up to US$1bn (€885,700) to fund President Donald Trump's wall on the Mexican border.

Patrick Shanahan authorised the commander of the US Army Corps of Engineers to begin planning and executing the funding.

This is to be done in support to the Department of Homeland Security (DHS) and Customs and Border Protection.

The US Department of Defense said in a statement: "These funds will be used to support DHS's request to build 57 miles of 18-foot-high pedestrian fencing, constructing and improving roads".

It is also to be used to install lighting within sectors of the border.

This follows the declaration of a 'national emergency' by President Trump on February 15th over the southern border of the United States.

The declaration gives the US Department of Defence authority to construct roads and fences and to install lighting to block drug-smuggling corridors across international boundaries.

US Secretary of Homeland Security Kirstjen Nielsen marks the the completion of the first section of the border wall in October 2018 | Image via @SecNielsen on Twitter

Democratic senators have claimed in a letter that the Pentagon did not seek permission from the appropriate committees before notifying Congress of the funds transfer.

CNN reports the letter was signed by "every Democratic senator on the Senate Appropriations Committee's subcommittees on Defense and Military Construction, Veterans Affairs and Related Agencies".

It read: "We strongly object to both the substance of the funding transfer, and to the department implementing the transfer without seeking the approval of the congressional defense committees and in violation of provisions in the defense appropriation itself.

"As a result, we have serious concerns that the department has allowed political interference and pet projects to come ahead of many near-term, critical readiness issues facing our military."

Announcing the national emergency outside the White House in February, Mr Trump said there were "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 had been made by other presidents "many times before."

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

"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.

US President Donald Trump signs the first veto of his presidency in the Oval Office of the White House, overruling Congress to protect his emergency declaration for border wall funding | Image: Evan Vucci/AP/Press Association Images

The declaration was made after he failed to secure $5.7bn (€5.04bn) in funding for the wall from the US Congress.

Democrats, who control the House of Representatives, insisted Mr Trump's declaration was unlawful and unconstitutional.

The Congress voted to oppose his decision to declare a national emergency.

But this was then vetod by Mr Trump - his first such use of a veto measure.

A number of legal challenges have also been launched by states and civil rights organisations, in a bid to end the controversial emergency.