The Pentagon has authorised $1bn to begin construction of a border wall on the US-Mexico border.
It will be used to build 92km of fencing and improve roads.
President Donald Trump has called the situation at the Mexican border a "crisis" and insists a physical barrier is needed to stop criminals crossing into the US.
Mr Trump issued the first veto of his presidency earlier this month, overruling Congress to protect his emergency declaration for border wall funding.
Acting US Defense Secretary Patrick Shanahan authorised the funding, which prompted anger from the opposition.
A statement on the Department of Defense website read: "Acting Secretary of Defense Patrick M. Shanahan authorized the commander of the U.S Army Corps of Engineers to begin planning and executing up to $1 billion in support to the Department of Homeland Security (DHS) and Customs and Border Patrol.
"These funds will be used to support DHS’s request to build 57 miles of 18-foot-high pedestrian fencing, constructing and improving roads, and installing lighting within the Yuma and El Paso Sectors of the border in support of the February 15 national emergency declaration on the southern border of the United States."
Democratic senators complained that the Pentagon had not sought permission from the appropriate committees before notifying Congress of the fund's transfer.
A number of the senators signed a letter to object to the funding, as reported by CNN.
It read: "We strongly object to both the substance of the funding transfer, and to the Department implementing the transfer without seeking the approval of the congressional defense committees and in violation of provisions in the defense appropriation itself."