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They have rifles and pistols but the Texas National Guard troops on the Rio Grande are on the border with Mexico only as physical deterrents and observers, a soldier told CNN on Tuesday from an observation post overlooking the river.

Members of the Arizona National Guard listen to instructions on April 9, 2018, at the Papago Park Military Reservation in Phoenix. (Credit: Caitlin O'Hara / AFP / Getty Images)
Members of the Arizona National Guard listen to instructions on April 9, 2018, at the Papago Park Military Reservation in Phoenix. (Credit: Caitlin O’Hara / AFP / Getty Images)

Troops are providing an extra set of eyes and ears — in this case as many as 12 extra sets — to Border Patrol agents, said a captain, who asked not to be named for security reasons.

“Essentially our guys are out here as a physical deterrent operation and to report to our local law enforcement partners of what it is that we see (as far as) any illegal activities,” the captain said.

On a warm and humid afternoon, the captain showed CNN two observation posts where troops, equipped with binoculars and a long-range surveillance system, scanned the river separating the United States and Mexico after President Donald Trump ordered thousands of troops to the border to increase security.

The captain told CNN that about four minutes after the media convoy left the first observation point, soldiers noticed a raft coming across the water. The Guardsmen radioed local law enforcement, including Border Patrol, who took it from there. It wasn’t immediately clear whether the people on the raft were doing anything wrong.

But the captain said about the mission, “if they spot anything that may potentially make it across the river, it is safe to say it’s probably an illegal activity.”

The captain wasn’t able to speak about operational details, like when rules of engagement would allow soldiers to use their weapons. He said he wasn’t aware of Texas National Guard troops having to use their firearms in any previous US deployments along the border.

But “we want our troops to be safe out here,” the captain said.

These troops are part of a contingent of 250 soldiers and airmen initially deployed. Texas Governor Greg Abbott announced Monday the state is preparing to increase its National Guard presence to more than 1,000 troops along the state’s 1,254-mile long border with Mexico. He said troop strength will grow by about 300 troops per week until all department of homeland security requirements are met.

Not all the troops will be at observation points or need to carry self-defense weapons, officials say.

US law limits what the troops can do. Federal law prohibits the military from being used to enforce laws, meaning troops cannot actually participate in immigration enforcement. Officials tell CNN many will serve in additional supporting roles to the Border Patrol, like training, construction and intelligence gathering.

Trump said last week he will deploy 2,000 to 4,000 troops to the US border with Mexico.

Sending National Guard troops to the Mexico border is not unprecedented. Both of Trump’s predecessors did so, though the moves were criticized as costly and of limited effectiveness.