More soldiers will be sent each Monday until 532 have joined the mission, said 1st Lt. Valentine Castillo, a National Guard spokesman in Phoenix.
"Everything is right on track to be fully operational by the beginning of October," Castillo said.
The soldiers got two to three weeks of training in surveillance techniques and first aid. They will be armed for self-defense, Castillo said, but will not have law enforcement authority. Instead, they will serve as "extra eyes and ears" for the U.S. Border Patrol.
Some will be assigned to teams in concealed locations to watch for smugglers and illegal immigrants entering the U.S. When the soldiers spot them, they'll call in agents to grab the suspects, Castillo said. Others will monitor computers and work with electronic detection systems.
"We're here to support Customs and Border Protection," he said.
Mario Escalante, of the Border Patrol's Tucson sector, said many will be doing mobile or remote surveillance. "It's going to help a lot," he said. "There will be certain areas where they play a major role as a deterrence factor," forcing illegal migrants to use routes patrolled by agents.
In March 2009, Brewer wrote to the Defense Department requesting 250 additional soldiers for Arizona's Joint Counter Narco-Terrorism Task Force, which already uses National Guard personnel.
Brewer spokesman Paul Senseman said the governor is "grateful for the additional assistance" but believes 6,000 reinforcements are now needed, with half of them in Arizona.
Senseman said Brewer is calling for the larger force to ensure that extreme cartel violence in northern Sonora does not leapfrog into Arizona.
Contributing: The Associated Press