WASHINGTON (AFP) - Acting Pentagon chief Patrick Shanahan said Monday (March 25) he had authorised US$1 billion (S$1.35 billion) 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 57 miles (92km) of 18-foot (5.5m) fencing, build and improve roads, and install lighting to support Mr Trump's emergency declaration as concerns the border.

Mr Shanahan "authorised the commander of the US Army Corps of Engineers to begin planning and executing up to US$1 billion in support to the Department of Homeland Security and Customs and Border Patrol," a Pentagon statement read.

The acting defence secretary 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.

The White House has laid out an ambitious 2020 budget proposal which contains US$8.6 billion in new wall funding, above the US$5.7 billion Mr Trump sought for this year.

Frustrated by Congress's refusal to provide the budget he wanted, Mr Trump declared a national emergency last month.

The White House has signaled it will seek to repurpose some US$6 billion from military funds, without specifying which Pentagon programmes would be slashed.

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.

MILITARY MISGIVINGS

There have also been reported misgivings within the military, including from America's top marine who last week warned that deployments to the US-Mexico border pose an "unacceptable risk" to the force, according to documents obtained by The Los Angeles Times.

In memos addressed to acting Pentagon chief Patrick Shanahan and Navy secretary Richard Spencer, General Robert Neller wrote that he had been forced to cancel or reduce exercises in five countries.

Gen Neller added the declaration meant the corps could not afford to rebuild hurricane-hit bases in North Carolina and Georgia.

"The hurricane season is only three months away... and we have Marines, Sailors, and civilians working in compromised structures," Gen Neller wrote.

COURT SLOWDOWN

The declaration has also been challenged by 16 states which sued the administration last month, contending the order was contrary to the constitution's presentment and appropriations clauses, which outline legislative procedures and define Congress as the final arbiter of public funds.

The lawsuit also questioned Mr Trump's categorisation of illegal border crossings as a national emergency, saying data issued by the administration itself refuted the notion.
Should the states prevail, the case could work its way up to the Supreme Court, setting up a precedent-setting showdown on the separation of powers.