The Pentagon has signed off on $1 billion for the U.S Army Corps of Engineers to support the Homeland Security Department and Customs and Border Protection in constructing a wall along the U.S.-Mexico border, the Department of Defense announced Monday.

Acting Defense Secretary Pat Shanahan approved the funds to go toward erecting 57 miles of 18-feet-high pedestrian fencing, and constructing and improving roads. The funds will also but used for installing lighting within the Yuma and El Paso sectors of the border.

The statement from the Defense Department said the projects are specifically catered to stopping drug-smuggling corridors.

President Trump declared a national emergency in February at the border after Congress refused to allocate $5.7 billion for his border wall project. The declaration moved billions of dollars from the military construction budget. Trump's move prompted a swift legal challenge from more than a dozen states, as well as rebuke from Democrats and some Republican lawmakers.

The $1 billion payment is the Pentagon's first transfer of funds designated to the border wall, but another $1.5 billion transfer is expected in the near future.

Earlier Monday, the Pentagon notified Congress of the money transfer to the U.S. Army Corps of Engineers, prompting backlash from all Democrats from the Senate Appropriations Subcommittee on Defense and Military Construction, Veterans Affairs, and Related Agencies. They noted the Pentagon did not consult the committee beforehand.

"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," the senators wrote in a letter to Shanahan, according to CNN. "As a result, we have serious concerns that the Department has allowed political interference and pet projects to come ahead of many near-term, critical readiness issues facing our military."

Those who signed the letter include Sens. Patrick Leahy of Vermont, Dick Durbin of Illinois, Brian Schatz of Hawaii, Chris Murphy of Connecticut, Tammy Baldwin of Wisconsin, and Dianne Feinstein of California, among others.