The authorisation was made by acting Pentagon chief Patrick Shanahan.
The Pentagon has authorised $1 billion for US President's Donald Trump's wall along the US-Mexico border. The authorisation was made by acting Pentagon chief Patrick Shanahan.
Shanahan "authorized the commander of the US Army Corps of Engineers to begin planning and executing up to $1 billion in support to the Department of Homeland Security and Customs and Border Patrol," a Pentagon statement read.
The statement was released just hours before Shanahan was due to testify in Congress to present and defend the Pentagon's draft budget.
The acting defence secretary defended the move by citing a federal law that he said "gives the Department of Defense 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 $1 billion will go towards building 92 kilometres of 18-foot-high fencing, building and improving roads, and installing lighting along sections of the border.
Trump had in December last year shut down the US government for 35 days — the longest in US history — after the Democrats refused to sign off on a funding bill giving him $5.7 billion for his wall.
In February, Trump declared a national emergency at the border — he said illegal immigration from Mexico was an "invasion" — which would allow the government to route funds meant for emergencies towards building the border wall. Trump is looking to unlock as much as $8 billion via the emergency funding route.
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.