The U.S. Department of Defense shifted $1 billion to plan and build a 57-mile section of "pedestrian fencing", roads and lighting along the border between the United States and Mexico, the Pentagon chief said on Monday.

Last week, the Pentagon gave Congress a list that included $12.8 billion of construction projects for which it said funds could be redirected for construction along the U.S.-Mexico border.

U.S. President Donald Trump declared a national emergency last month in a bid to fund his promised border wall without congressional approval.

Acting Secretary of Defense Patrick Shanahan said in a memo to Secretary of Homeland Security Kirstjen Nielsen that the Department of Defense had the authority to support counter-narcotics activities near international boundaries.

Shanahan authorized the U.S Army Corps of Engineers to begin planning and executing the project that would involve building 57 miles of 18-foot-high fencing, constructing and improving roads, and installing lighting within the Yuma and El Paso sections of the U.S.-Mexico border.