President Donald Trump's deployed National Guard troops have already begun arriving at the U.S.-Mexico border — and they'll mostly be providing aerial support and helping with surveillance and infrastructure projects, the Department of Defense said Monday.

But the troops are explicitly barred from helping arrest or deport immigrants, as the Posse Comitatus Act of 1878 limits the military's ability to enforce civilian law without authorization.

The troops are set to use drones and light-, medium-, and heavy-lift helicopters during their deployment, Pentagon spokesman Lt. Col. Jamie Davis told The Washington Post in a statement. They'll also assist with surveillance systems such as cameras and blimps.

Beyond that, the troops will be doing maintenance work on roads and facilities, as well as clearing vegetation, Davis said. He did not clarify whether those infrastructure tasks would include border wall construction.

Related: Mexico Is Not Happy With Trump’s Plan to Send 4,000 US Troops To The Border »

The Pentagon confirmed in a statement last week that the troops won't conduct law-enforcement activities or interact with migrants or detainees without approval from Defense Secretary Jim Mattis.

Davis also said the troops won't be conducting armed patrols, and will only carry weapons in limited circumstances, depending on their mission.

“National Guard personnel will only be armed for their own self-protection to the extent required by the circumstances of the mission they are performing,” he said.

It's still unclear exactly how many troops will be deployed to the border — though Texas, Arizona, and New Mexico have so far committed nearly 1,600 members altogether. Trump said last Thursday he hoped the states' governors would authorize “anywhere from 2,000 to 4,000.”

The 1 Thing The National Guard Won’t Do At The US-Mexico Border

President Donald Trump visits Border Wall prototypes in San Diego, California, on March 13, 2018.U.S. Customs and Border Protection Office of Public Affairs/Ralph Desio

The only border state that hasn't yet responded to the Trump administration's request is California, whose Democratic Gov. Jerry Brown has been a vehement critic of Trump and his anti-immigration agenda.

It's also unclear what the deployments will cost and how long they'll last, though Mattis has already authorized a payment that would cover the cost of up to 4,000 National Guard members through September 30.

Trump's demand to have the National Guard deployed along the border came after a days-long tirade against a “caravan” of hundreds of central American migrants traveling through Mexico. Some of those migrants intended to seek asylum in the United States or enter illegally.

Though the caravan has mostly dispersed, organizers said Monday that roughly 200 migrants still intend to claim asylum in the US.

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