US Acting Defence Secretary Patrick Shanahan approved up to US$1 billion for building a portion of the wall along the US-Mexico border.

Shanahan authorized 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, the Pentagon said in a statement late Monday.

The Pentagon said the funds will be used to build 92 kilometres of 5.5-metre high pedestrian fencing, construct and improve roads, and install lighting within the Yuma and El Paso sectors of the border.

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

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.

The move aims to support the Feb 15 national emergency declaration on the southern border of the United States, it added.

President Donald Trump moved to build his long-promised border wall through an executive order last month after he failed to get the US$5.7 billion he was seeking from Congress for the barrier's construction, having shut down Congress for the longest stretch in history over his demand.

Opponents of Trump's executive order have warned that it sets a dangerous precedent and violates the Constitution's delegation of funding powers to the legislative branch.

Senator Chuck Schumer, the top Senate Democrat, ripped Trump's action in a statement, saying "it is no surprise that the president holds the rule of law and our Constitution in minimal regard". 

- Agencies