US troops detain immigrants in border defence zone as military role expands

US troops have begun directly detaining immigrants accused of trespassing on a recently designated national defence zone along the southern US border, in an escalation of the military's enforcement role, authorities said on Wednesday.

US Army Lieutenant Colonel Chad Campbell described in detail the first detentions by troops last week of three immigrants accused of trespassing in a national defence area near Santa Teresa, New Mexico.

Those migrants were quickly turned over to US Customs and Border Protection and are now among more than 1,400 migrants to have been charged with illegally entering militarised areas along that border, under a new border enforcement strategy from President Donald Trump's administration.

Troops are prohibited from conducting civilian law enforcement on US soil under the Posse Comitatus Act. But an exception known as the military purpose doctrine allows it in some instances.

Authorities "noticed three individuals crossing the protective barrier into the United States," Campbell said. "A Department of Defense response went to interdict those three individuals, told them to sit down. ... In a matter of three minutes, border patrol agents came in to apprehend. So that three minutes is that temporary detention" by the military.

Trump has designated two national military defence areas along the southern US border for New Mexico and a 97-kilometre stretch of western Texas, from El Paso to Fort Hancock, while transferring much of the land from the Interior Department to oversight by the Department of Defense for three years.

The Trump administration plans eventually to add more militarised zones along the border, a military spokesman said on Wednesday at a news conference in El Paso.

"We have been very clear that there will be additional National Defence Areas across the southern border," said Geoffrey Carmichael, a spokesperson for an enforcement task force at the southern border. "I won't speculate to where those are going to be."  Proponents of the militarised zones, including federal prosecutors, say the approach augments traditional efforts by Customs and Border Protection and other law enforcement agencies to secure the border.

"These partnerships and consequences exist so that we can promote the most humane border environment we've ever had," El Paso sector Border Patrol Chief Agent Walter Slosar said. "We are dissuading people from entering the smuggling cycle ... to make sure that smugglers cannot take advantage of individuals who are trying to come into the United States."

Defence attorneys and judges in some instances are pushing back against the novel application of national security charges against immigrants who enter through those militarised zones and carry a potential sentence of 18 months in prison on top of a possible six-month sentence for illegal entry.

A judge in New Mexico has dismissed more than 100 national security charges against immigrants, finding little evidence that immigrants knew about the national defence areas. Those migrants still confronted charges of illegal entry to the US.

In Texas, a Peruvian woman who crossed the US border illegally was acquitted of unauthorised access to a newly designated militarised zone in the first trial under the Trump administration's efforts.

US Attorney Justin Simmons, who oversees western Texas, vowed to press forward with more military trespassing charges.

"We're gonna keep going forward on these NDA charges," Simmons said. "We are gonna still bring them, we may win on them, we may not. ... At the end of the day, you are not going to be allowed to stay in this country if you enter this country illegally."  Greater military engagement at the border takes place at the same time dozens of mayors from across the Los Angeles region banded together on Wednesday to demand that the Trump administration stop the stepped-up immigration raids that have spread fear across their cities and sparked protests across the US.

Trump has authorised the deployment of an additional 2,000 National Guard members to respond to immigration protests in LA. That directive brings the total number of Guard put on federal orders for the protests to more than 4,100. The Pentagon had already deployed about 700 Marines to the protests to the city.