The Pentagon has relocated $1bn to army engineers to build a portion of the wall along the US-Mexico border, acting Defense Secretary Patrick Shanahan has said.
Shanahan said late on Monday 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 authorised the engineers to begin planning and executing the project that would involve building 98 kilometres of five-metre-high fencing, constructing and improving roads, and installing lights within the Yuma and El Paso sections of the US-Mexico border.
The infrastructure was described as "fencing", not a "wall".
Building a wall along the two country's border was a key campaign promise for Republican President Donald Trump in the 2016 election. The opposition Democrats have vehemently opposed the efforts to build a wall.
After weeks of bickering, Republicans and Democrats agreed to a compromise during budget talks in February that saw Trump allocate $5.7bn for building a wall - much less than he had hoped for.
In response, Trump declared a national emergency last month, clearing the way for the president to take money from the Pentagon's construction budget and drug forfeitures and use it for wall construction.
Trump overruled a move by Congress to block the declaration of a national emergency on March 15, using the first veto of his presidency.
Democrats are hoping to block the veto in a vote on Tuesday, but they are not expected to muster the two-thirds majority required to do so.
Even if they are successful, they would require that same majority in a second vote in the Republican-controlled Senate.