SANTA FE, N.M. — Immigrants recently detained in southernmost New Mexico now face a novel criminal charge of breaching a national defense area, after the U.S. Army assumed oversight of a 170-mile (274-kilometer) strip along the southern U.S. border in cooperation with immigration authorities.

Federal prosecutors on Monday applied the additional charge for incursions into the recently designated New Mexico National Defense Area against migrants detained by Customs and Border Protection, as the military scales up troop deployments to a sliver of U.S. borderlands that is now being treated as an extension of U.S. Army Garrison Fort Huachuca in Arizona.

The Trump administration says those soldiers have the authority to temporarily apprehend trespassers, amid efforts to get around a federal law that prohibits U.S. troops from being used in domestic law enforcement on American soil.

Defense Secretary Pete Hegseth highlighted the changes Friday while visiting troops at the New Mexico border.

“Any illegal attempting to enter that zone is entering a military base, a federally protected area,” he said alongside a border wall, in a video posted social media. “You will be interdicted by U.S. troops and Border Patrol.”

New Mexico-based ACLU attorney Rebecca Sheff warned that the military buffer zone “represents a dangerous erosion of the constitutional principle that the military should not be policing civilians.” She expressed concern that U.S. citizens that live near the border could be prosecuted under the same provisions.

The charges against at least a half-dozen immigrants for unauthorized entry on military defense property were signed by U.S. Attorney Ryan Ellison, an Alamogordo, New Mexico-native sworn into office April 18.

Troops are prohibited from conducting civilian law enforcement on U.S. soil under the Posse Comitatus Act. An exception known as the military purpose doctrine allows it in some cases.

The newly militarized corridor includes the Roosevelt Reservation, a 60-foot-wide (18-meter-wide) federal buffer zone that ribbons along the border, except where it encounters tribal or privately owned land.

Control of the Roosevelt Reservation was transferred in mid-April from the Interior Department to the Defense Department in a presidential memo. The Interior Department also has designated areas beyond the Roosevelt Reservation for transfer to military oversight.

Since then, the Army has announced several military deployments to augment surveillance, expand roadways and shore up barriers at the border.

Gonzalez reported from McAllen, Texas.