The Department of Defense will allocate up to $1 billion in funding to build President Trump's wall along the US–Mexico border, Acting Defense Secretary Patrick Shanahan said in a letter Monday night.

Shanahan wrote to Homeland Security Secretary Kirstjen Nielsen that military funding would be provided for construction along the border to combat drug smuggling. DHS had requested military funding last month to construct roads, lighting, and fencing at 11 project sites along the border.

Shanahan agreed to provide funds up to $1 billion for three of the projects suggested by the Department of Homeland Security, which he described as constructing 57 miles of 18-foot-high pedestrian fencing, roads, and lighting in the El Paso, Texas, and Yuma, Arizona, sections of the border.

But Democrats on the House Armed Services Committee are seeking to block that funding transfer. Rep. Adam Smith, the committee chair, told the Department of Defense in a letter that the committee denied the plan — though it wasn't immediately clear if members of Congress could stop the funding transfer.

"The committee does not approve the proposed use of Department of Defense funds to construct additional physical barriers and roads or install lighting in the vicinity of the United States border," Smith wrote.

Democrats in Congress also on Tuesday failed to override President Trump's veto regarding a national emergency at the border. The national emergency, declared by the president in February, will now continue.

That national emergency has already threatened the combat readiness of the Marine Corps, commandant Gen. Robert Neller said last week. He cited the deployment of troops to the border as well as the diversion of funding toward border construction.

Senate Democrats have also questioned the impact of diverting defense funds toward border construction. Shanahan previously provided a list of military construction projects that could be put on hold because of the transfer of funding to the border.

“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,” a letter signed by 10 Democrat senators read. “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.”

Projects on the list ranged from a flight simulator at an Air National Guard station in California to a water treatment facility at North Carolina's Camp Lejeune.

In his statement, Smith said he looked forward to questioning Shanahan at the committee hearing.

"We look forward to hearing how he intends to pilfer the military construction accounts, circumvent the intended nature of the law, while simultaneously abusing the trust of the American people,” he said.