Acting Pentagon chief Patrick Shanahan has authorised $1 billion (£757 million) to build part of the wall sought by Donald Trump along the US-Mexico border, the first funds designated for the project under the president's emergency declaration.

The Department of Homeland Security asked the Pentagon to build 57 miles (92 kilometres) of 18-foot (5.5-metre) fencing, construct and improve roads, and install lighting to support Mr Trump's emergency declaration.

Mr 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 issued late Monday read.

The acting defence secretary cited a federal law that he said gives the Pentagon broad authority to build infrastructure "across international boundaries of the United States in support of counter-narcotic activities of federal law enforcement agencies."

The statement was released a day before Mr Shanahan was due to testify in Congress to present and defend the Pentagon's draft budget.