A senior defense official says the Pentagon is reviewing how U.S. soldiers responded during an incident this month in which Mexican troops detained and disarmed Americans on Texas soil.

The standoff between two U.S. soldiers and as many as six Mexican military officials on April 13 is believed to be the first of its kind, according to the senior defense official from Northern Command, or NORTHCOM. "This is the first incident that we're aware of that the two militaries came together," the official told the Washington Examiner.

Two Army soldiers from Washington state were sitting in an unmarked Customs and Border Protection vehicle south of the U.S. barrier but north of the international boundary near Clint, Texas, when Mexican troops moved in on them.

The Mexican soldiers, each carrying FX-05 Xiuhcoatl rifles, detained, disarmed, and questioned the U.S. troops. One soldier's Beretta M9 service pistol was taken from him and temporarily confiscated.

The Pentagon is now investigating the incident, which the official said "will help us modify any instructions that we're giving the troops" about how to deal with such a situation.

Troops deployed to the U.S.-Mexico boundary go through joint readiness staging, or training on how to handle dangerous situations in the area. The official said he could not recall anything similar to last Saturday's encounter having taken place during a previous active-duty troop deployment.

No official protocol exists for how to navigate a run-in with a foreign military, but the senior official said the soldiers were trained to "de-escalate" the situation. By surrendering at least one gun, they followed existing protocol, though it left them unarmed.

The NORTHCOM official also defended the U.S. soldiers being in the location. The pair had been assigned by Customs and Border Protection to be at those coordinates on the U.S. side of the border. The two soldiers were one of 150 teams serving on mobile surveillance missions who had been assigned that specific location to stake out and monitor surveillance feeds.

Mexican soldiers spotted the pair and did not recognize their unmarked vehicle. The U.S. troops did not recognize the unmarked truck. There was mutual confusion about why either party was at that location.

"That area of the border is kind of confusing," a second NORTHCOM official told the Examiner. "It may have been difficult for them [Mexican forces] to know if they didn't know the area as well or were new or something. I don't think — it definitely wasn't trying to overtake the U.S."

Much of the physical barrier along the U.S.-Mexico border does not sit on the international boundary and is located a few dozen to a few hundred feet north of it.

In areas such as southwestern Arizona and eastern Texas, rivers serve as the official border, but in other regions, it can be more difficult to determine the official line in the sand.

The language barrier further complicated the situation. "There was a U.S. Army soldier that was one of the two that spoke Spanish. That was about when they came to realize they were Mexican military," the official said.