Trump expands military role at southern border, authorises detention of people who enter illegally

The Department of Defense is expanding a militarised zone along the southern US border where troops are authorised to detain people who enter illegally for possible federal prosecution on charges of trespassing in a national defence area.

The Air Force announced Monday the annexation of a serpentine 250-mile (400-kilometre) stretch of the border in Texas amid a buildup of military forces under President Trump's declaration of a national emergency at the border. A Defense Department official said the Navy also has been instructed to establish a new national defence area at the border. The official didn't provide further details.

The newly designated national defence area on land and water along the Rio Grande spans two Texas counties and runs alongside cities including Brownsville and McAllen. It will be treated as an extension of Joint Base San Antonio. The Air Force said it's prepared to install warning signs immediately against entry to the area.

The military strategy was pioneered in April along a 170-mile (275-kilometre) stretch of the border in New Mexico and expanded to a swath of western Texas in May. Hunters, hikers and humanitarian aid groups fear that they will no longer have access.

In the newest national defence area, military responsibilities include "enhanced detection and monitoring" and "temporarily detaining trespassers until they are transferred to the appropriate law enforcement authorities," the Air Force said in a statement.

At least three people have been directly detained by troops in New Mexico for processing by Border Patrol. More than 1,400 immigrants have been charged with incursions into the national defence areas, a criminal misdemeanour punishable by up to 18 months in prison.

Court challenges to the charges have been met with mixed results.

The militarised border zone is a counterpoint to the deployment of roughly 4,000 National Guard troops and 700 Marines to Los Angeles following protests over Trump's stepped-up enforcement of immigration laws.

The troop deployments are testing the limits of the Posse Comitatus Act, which prohibits the military from conducting civilian law enforcement on US soil.

Arrests at the border for illegal entry have decreased dramatically this year.