By Natasha Bertrand, Priscilla Alvarez, Haley Britzky, Oren Liebermann and Katie Bo Lillis

(CNN) — Thousands of additional active duty US troops are being ordered to the southern US border with Mexico, just two days after President Donald Trump mandated that the US military step up its presence there, according to officials familiar with the matter.

There are already roughly 2,200 active duty forces at the border as part of Joint Task Force-North, US Northern Command’s border mission based out of El Paso, Texas. They help support US Customs and Border Protection’s work there, performing mostly logistical and bureaucratic tasks like data entry, detection and monitoring, and vehicle maintenance.

It is not yet clear which specific units are being ordered to the border, but an initial wave will include around 1,500 troops.

“First operations for them should commence within the next 24-48 hours, they’re moving right now, as we sit here,” a senior military official told reporters on Wednesday.

Acting Secretary of Defense Robert Salesses said he directed the Defense Department to “begin augmenting its forces at the southwest border” with 1,500 ground personnel “as well as helicopters with associated crews, and intelligence analysts to support increased detection and monitoring efforts.”

Salesses also confirmed CNN’s earlier reporting that US Transportation Command had been instructed to prepare to use US military assets, including military aircraft, for migrant repatriation flights.

The senior military official said four total aircraft — two C-17s and two C-130s — are being sent to San Diego and El Paso to support repatriation flights. The air crews for those aircraft are not included in the 1,500 ground troops being sent.

“[T]he Department will provide military airlift to support DHS deportation flights of more than five thousand illegal aliens from the San Diego, California, and El Paso, Texas, sectors detained by Customs and Border Protection,” Salesses said in a statement. “DHS will provide inflight law enforcement, and the State Department will obtain the requisite diplomatic clearances and provide host-nation notification.”

“This is just the beginning,” Salesses added.

White House press secretary Karoline Leavitt also confirmed the number to reporters at the White House on Wednesday.

“President Trump signed an executive order - 1500 additional troops to United States southern border. This comes off of his day-one action…to direct the Department of Defense to make homeland security a core mission of the agency,” Leavitt said.

The Trump administration asked the military earlier this week to be prepared to deploy up to 10,000 active duty troops immediately, setting off a scramble inside the Pentagon, one of the officials and another person familiar with the matter told CNN. Military officials have pushed back on that because they believe that sending so many troops to the border at once could pull them away from other mission requirements elsewhere in the world and strain resources, the first official said.

Even more active duty troops are expected to be deployed to the border in the coming weeks and months, one of the officials said, with this first wave laying the groundwork for a larger military footprint.

A senior defense official denied on Wednesday that the 10,000 number had been floated, though the senior military official said 10,000 “could be the number … but we don’t know, it’s kind of too early to tell as we’re in the planning process with our interagency partners.”

The senior military official also said that the initial 1,500 were chosen because it would not have an impact on other mission or deployment requirements, and as the department looks at potentially deploying more troops, “we’ll make that judgement” regarding potential risk to other operations or troop readiness.

“That’s really a big part of our job, is to look at what is risk to mission, risk to force, over the set of global priorities,” the military official said.

The initial wave of 1,500 troops includes 500 Marines from Camp Pendleton and the 1st Marine Division in San Diego, according to another defense official. The Marines are expected to arrive at the border imminently and will be prepared for all contingency operations, acting as both a rapid response force and a long-term solution.

“We’ve been told to treat this like a national emergency because it’s been declared a national emergency,” the official said. “Don’t be surprised if you see Marines being dropped off by helicopters.”

The number of Marines deployed as part of the border mission could ultimately climb to 2,500, the official said. The senior military official said the Marines had been on call to support the response to the California wildfires but had been released from that mission as they were no longer needed.

The senior military official said the 1,500 also includes roughly 1,000 Army personnel including “a battalion headquarters.” Those personnel will largely be military police, another Pentagon official told CNN, who are coming from a patchwork of Army installations in the US, to include Forts Bliss, Riley, Cavazos, Stewart, Carson, and Campbell.

Already at the border are UH-72 Lakota helicopters that have been supporting Customs and Border Protection; the senior military official said they flew their first missions on Wednesday.

There is also a National Guard contingent at the border called Operation Lonestar, headed by the Texas National Guard. There are roughly 4,500 National Guardsmen currently assigned to the mission, according to the Texas Military Department.

The additional active duty troops being sent to the border this week will be doing much of the same, the officials said, and are expected to feed into and augment Joint Task Force-North.

They will be helping to maintain operational readiness for Border Patrol, assisting in command-and-control centers, and providing more intelligence specialists to assess threats and migrant flows, according to sources familiar with the planning.

The troops are also expected to augment air assets and help with air operations.

It is not clear whether the troops will be armed. But none of the active duty troops are authorized to perform any kind of law enforcement role, like perform arrests or seize drugs, or engage with migrants other than to help transport them to and around different migrant facilities.

A law that dates back more than a century known as “posse comitatus” bars active-duty US troops from domestic law enforcement without authorization. Other laws and regulations have further clarified that troops can’t participate in activities such as making arrests and conducting searches, according to a Congressional Research Service analysis.

President Trump did say in an executive order Monday, however, that he will decide within 90 days whether to invoke the Insurrection Act at the US-Mexico border, which would allow him to use active duty troops domestically for law enforcement.

Federal resources along the US southern border have been stretched thin for years amid an influx of migrants.

The number of migrants crossing the southern border has dropped recently—there are between 1,100 to 1,300 migrants illegally crossing the border daily, according to a Homeland Security official.

But by adding more Pentagon personnel, sources expect Immigration and Customs Enforcement, which has been assisting along the border, will be able to go back to the interior to focus on arrests of undocumented immigrants already inside the US.

This story has been updated with additional details.

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