Curt Prendergast Reuters

3:29 p.m. CST, February 16, 2012

DOUGLAS, Arizona (Reuters) - U.S. authorities have upgraded six miles of border fencing in a remote Arizona ranching town with a taller barrier that will be tougher to breach, in the latest effort to tighten their grip on the isolated stretches of the Mexico border.

The U.S. Customs and Border Protection agency said it has replaced outdated panel fencing that carved across the southern reach of Douglas, in the far southeast corner of Arizona, with an 18-foot-tall bollard and steel-mesh fence design.

The move to refurbish the barrier separating Douglas from Agua Prieta in Mexico's northern Sonora state follows an overhaul of 2.8 miles of fencing in Nogales, the largest city on the Arizona-Mexico border, last year.

"Every time we get a new technology it allows us to expand." said David Herrera, a spokesman for the U.S. Border Patrol's Tucson sector, of the overhaul completed this week.

"Securing cities like Douglas allows us the flexibility to move agents to remote areas if need be," he added.

The revamp in the high plains ranching town comes as arrests of illegal immigrants crossing north over the Mexico border plummeted to 327,577 last year, their lowest levels since 1972 whenPresident Richard Nixon was in the White House.

Factors in the stark decline have included tightened border enforcement, a slowed U.S. economy providing fewer jobs to undocumented workers, and increased drug cartel-related violence in Mexico that has made the journey north more hazardous, according to analysts.

The efforts to tighten security at smaller border cities like Douglas, which has around 17,000 residents, is coupled with moves to crack down on smuggling in some of the loneliest spaces in between them.

In the coming days, the U.S. Border Patrol is to start work on its third station in the boot heel of New Mexico further to the east where agents recorded just 6,910 arrests last year - fewer than 20 a day.

Opening the forward operating base in the Animas Valley later this year, with stabling for horses, a helicopter landing pad and living quarters for agents, marks a shift toward closing off less transited areas of the border, agents say.

"During the 1990s the focus was just on trying to gain greater manageability on metropolitan areas like El Paso," said Doug Mosier, the Border Patrol's spokesman for the El Paso sector, which also covers New Mexico.

"Now we are able to focus on outlying areas and new challenges that we're seeing," he added.

Although some local residents were upset that the site chosen for the base was 20 miles north of the border, rather than at another proposed site closer to the line, they said they were glad to have more Border Patrol agents in the region.

"The boot heel of New Mexico has been a forgotten area on the border," said Judy Keeler, a rancher in the Animas Valley which lies a few miles from the spot where an Arizona rancher, Robert Krentz, was shot and killed in 2010.

Police suspected illegal immigrants or smugglers in the killing, although no arrests have been made.

"We held a lot of meetings and everybody was singing the same song. We all wanted more manpower on the border," Keeler said.

U.S. tightens grip on remote Mexico border areas -