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The 1,200 National Guard troops President Obama is dispatching to the U.S.-Mexico border will provide intelligence, reconnaissance and other "back office" support, but one thing they won't be doing is helping a beleaguered Border Patrol and local law enforcement nab illegal immigrants and smugglers flowing into Arizona.

Speaking at a press conference in the White House East Room Thursday, the president said the Guard won't be on the front lines but will be offering vital support roles.

"What we find is, is that National Guards persons can help on intelligence, dealing with both drug and human trafficking along the borders. They can relieve border guards so that the border guards then can be in charge of law enforcement in those areas. So there are a lot of functions that they can carry out that helps leverage and increase the resources available in this area," he said.

"They can relieve border guards so that guards can be in charge of law enforcement," the president added.

The State Department on Wednesday described the border plan as "not about immigration," but rather about the unregulated flow of drugs and guns being sent into Mexico from the U.S.as well as humans streaming northward.

"And let's understand, we're talking about, you know, flows going in both directions. It's not about immigration. It's not about the flow of certain things coming in this direction. We recognize, as the president has said, as the secretary has said, you know, we have responsibilities here, both in terms of the demand for narcotics within our country, the flow of weapons from our country into Mexico that helps to fuel the violence that Mexico is struggling with," State Department spokesman Philip Crowley said.

Obama said his plan to send as many as 1,200 troops was shaped last year before a national controversy erupted over Arizona's tough new law cracking down on illegal immigrants. The law allows state police to determine whether an individual is in the state legally when he or she is questioned by police on other suspicions.

Arizona Gov. Jan Brewer repeatedly has said that she signed the law because the federal government has offered no assistance in closing up the gaping border.

Crowley said it's not the first time the Guard have been sent to the border to work in secondary roles.

"You know, the rationale is simply to be able to, you know, take some support functions, relieve the civilian law enforcement of certain support functions and establish a broader presence across our border with Mexico," he said. "This frees up some resources that can be used more effectively to directly interdict the flow of illegal drugs, you know."

U.S. Ambassador to Mexico Carlos Pascual, speaking to journalists Wednesday in Mexico City, said the troops will serve as a bridge until the American government can get more agents on the border. He emphasized that the troops won't be working on the front lines or interacting with people crossing the border.

"It's much more back office functions of receiving reports that are coming in from other intelligence entities," he said. The troops will "review and analyze" intelligence, then "feed that to the people who are actually the presence on the border itself." In addition, he said the troops will observe the border from remote surveillance points, then communicate with Customs and Border Protection officers.

On Thursday, the Senate voted down an amendment by Arizona Sen. John McCain, who proposed sending 6,000 National Guard troops to the border, saying 3,000 of his proposed sum should go directly to Arizona to help protect his state.

Obama said he wants more than just a border security approach to the nation's immigration problems.

"I don't see these issues in isolation, not solely as a problem of the National Guard," he said, noting he wants a bill in Congress that does "a good job of securing our borders, holds employers accountable, makes sure that those who have come here illegally have to pay a fine, pay back taxes, learn English and get right by the law."

Obama and top members of his administration have said they disapprove of Arizona's law, and the Justice Department is weighing civil action against the state. The president would not condemn boycotts of the state over its law, saying it is for private citizens to decide, not the president of the United States.

"You know, I'm the president of the United States. I don't endorse boycotts or not endorse boycotts," he said. "What my administration is doing is examining very closely this Arizona law and its implications for the civil rights and civil liberties of the people in Arizona, as well as the concern that you start getting a patchwork of 50 different immigration laws around the country in an area that is inherently the job of the federal government."

The Associated Press contributed to this report.