The U.S. military will support deportation flights and intelligence collection and help construct barriers when 1,500 active-duty troops deploy to the southern border and begin work in the next two days. In a statement, acting Defense Secretary Robert Salesses said the deployment mission began on Wednesday and involves military airlift support for the Department of Homeland Security (DHS) to help deport more than 5,000 undocumented immigrants detained in San Diego, California and El Paso. U.S. troops will also assist in constructing both temporary and permanent physical barriers at the border, according to the statement. "This is just the beginning," Salesses said. "In short order, the Department will develop and execute additional missions in cooperation with DHS, federal agencies, and state partners to address the full range of threats outlined by the president at our nation's borders." A senior defense official said President Trump views the border as a "huge priority" and that these were just initial steps. "This is the initial effort that we can do right away, and ... we anticipate many additional permissions after this," the official told reporters. "This is just the start." The official said up to 10,000 troops could be deployed in future missions but cautioned that the figure was not final. Earlier Wednesday, White House press secretary Karoline Leavitt said Trump signed an executive order to deploy the 1,500 U.S. troops. "This is something President Trump campaigned on, the American people have [been] waiting for such a time as this, for our Department of Defense to actually take homeland security seriously," she said. "This is a number one priority of the American people, and the president has already delivered.” The troops slated for deployment include 1,000 Army soldiers and 500 Marines. The deployment includes additional helicopter crews and intelligence analysts, along with C-17 and C-130 transport planes. A senior military official said the Marines activated for the mission were on standby for deployment to the California wildfires, but that assignment was recently lifted as the fires have been more contained. The total number of troops at the border will be 4,000 once the additional forces arrive, according to the military official. While the troops are expected to be armed, "none of the forces that we're sending there are intended to be used for law enforcement," the military official said. U.S. Northern Command already has an active border defense mission called Joint Task Force North. Most of the troops under the task force are National Guard members, but active-duty service members can be deployed and have been in the past to assist at the border. U.S. troops primarily help with logistics, detection, monitoring, transportation and assisting Customs and Border Protection. Trump signed several executive orders on Inauguration Day to assert more control over the border and migration flows. One order deemed the situation at the border a national emergency to allow more troops to deploy, and another directed Northern Command to draft detailed operational plans to assist at the border. Salesses met Tuesday with Joint Chiefs of Staff Chair CQ Brown Jr.; senior policy leaders at the Pentagon, Northern Command and Transportation Command; and the chief of the National Guard Bureau. The Pentagon has set up a task force led by Northern Command to oversee the implementation of the border response.