Acting US Defence Secretary Patrick Shanahan has authorised $1bn to build part of the wall sought by President Donald Trump along the US-Mexico border.
The Department of Homeland Security asked the Pentagon to build 92km of 5.5-metre fencing, build and improve roads, and install lighting.
Mr Shanahan "authorised the commander of the US Army Corps of Engineers to begin planning and executing up to $1bn in support to the Department of Homeland Security and Customs and Border Patrol," the Pentagon said.
The acting Pentagon chief defended the move by citing a federal law that he said "gives the Department of Defence the authority to construct roads and fences and to install lighting to block drug-smuggling corridors across international boundaries of the United States in support of counter-narcotic activities of federal law enforcement agencies".
The statement was released just hours before Mr Shanahan was due to testify in Congress to present and defend the Pentagon's draft budget.
Meanwhile, despite strong opposition to the Pentagon’s move, The US House of Representatives has failed to override President Donald Trump's first veto, leaving in place the national emergency he declared last month to build a US-Mexico border wall that Congress has not funded.
Democrats who control the House did not attract enough Republican support to reach the two-thirds majority vote needed for an override. With the 248-181 vote, President Trump can continue scouring federal accounts for money he wants redirected to wall construction projects.
The White House has laid out an ambitious 2020 budget proposal, which contains $8.6bn in new wall funding, above the $5.7bn Mr Trump sought for this year.
Frustrated by Congress's refusal to provide the budget he sought to build a border wall, Mr Trump declared a national emergency last month in order to unlock funding without congressional approval.
The move drew condemnation from both the president's rival Democrats and fellow Republicans, who warned it was an abuse of presidential powers and created a dangerous precedent.
Mr Trump has made border security an over-arching domestic issue and says it will remain at the centre of the agenda in his 2020 reelection bid.
Although there has been a surge in arrival of families and children at the border, overall apprehensions at the frontier are down substantially from a decade or more ago.
