PHOENIX - President Barack Obama's plan to send as many as 1,200 National Guard troops back to the U.S.-Mexico border quelled demands that he must do more to battle illegal immigration and drug smuggling, but advocates for tougher enforcement say the troops need authority to make detentions.

The new plan looks similar to the National Guard initiative under former President George W. Bush, but on a much smaller scale: Troops will work on intelligence, surveillance and reconnaissance support, and will eventually be replaced by more border patrol and customs agents. The plan at this point doesn't call for the ability to round up suspected illegal immigrants and smugglers.

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.

The comments came a day after the Mexican government issued a statement saying it hoped the troops would be used to fight drug cartels and not enforce immigration laws. Mexico has traditionally objected to the use of the military to control illegal immigration.

Under Bush, the National Guard troops were designed to back the Border Patrol for two years as 6,000 more agents were trained and hired, and they weren't allowed to detain immigrants or smugglers. They were pulled out in July 2008, as planned, but many argue that drug violence and immigrant smuggling continue unchecked.

Pascual said the U.S. learned from that operation. "The biggest lesson was that we needed a much bigger and stronger civilian law enforcement presence along the border," he said.

Arizona's sweeping new immigration law, which requires police to question anyone they suspect of being in the country illegally, has made the topic a national campaign issue. Obama was pushed to take action Tuesday after Republicans threatened to force a congressional vote on sending troops to the border.

Republican state Sen. Russell Pearce of Arizona, the author of the state's new immigration law, said he fears Obama will repeat what he sees as Bush's mistake in not giving troops the power to confront and detain violent smugglers and other armed criminals along the border.

Pearce was disturbed by an incident in 2007 where National Guard troops backed off and called in federal agents as gunmen approached their Arizona post.

While supporters of the decision said the Guard members did as they were supposed to, Pearce questioned the point of having troops on the border if they can't confront such dangers. "It was a welcome-wagon role last time," Pearce said. "They weren't allowed to do anything."

Obama's plan also calls for sending only a fifth of the 6,000 troops deployed under Bush. It is unclear where on the border the soldiers will be sent.

Cochise County Sheriff Larry Dever, whose jurisdiction includes about 80 miles (about 130 kilometers) of the Arizona-Mexico border, said 1,200 soldiers might make a difference along a smaller portion of the border. "But if you spread it across the border, it's like spitting into the wind," Dever said.

Arizona Attorney General Terry Goddard, a Democrat like Obama who has prosecuted drug and immigrant smugglers, said the planned deployment was a good first step, but believes that the president's plan should evolve to include more troops and more authority.

"I'll take what we can get," Goddard said. "Again, I don't think this is the final response."

Obama is also requesting $500 million for border protection and law enforcement activities.

When Bush sent the National Guard to the border, the presence of the troops had a chilling effect on smugglers and would-be border-crossers, especially at spots where soldiers could be seen peering into Mexico.

T.J. Bonner, president of the National Border Patrol Council, a union representing 17,000 agents, said he doesn't see the broad outlines of the Obama plan as a solution to border violence.

"People shouldn't be surprised if the violence continues," Bonner said. "They shouldn't expect that the announcement of up to 1,200 National Guard members will send a shock wave of fear in the cartels and that they will start playing nice."

Republican Rep. John Carter of Texas said 25,000 troops were needed on the border and the soldiers should be armed with orders to shoot if they are fired upon. "The next time a gunman raises a rifle in our direction these guys are ready and able to light 'em up," Carter said in a written statement. "Once that happens a couple times we'll start to see some law and order again."

Carter drew a historical parallel, citing President Woodrow Wilson's decision to send troops to the border and into Mexico to pursue Francisco "Pancho" Villa after his attack on the New Mexico border town of Columbus.

"President Wilson sent 100,000 National Guard in 1916 when violence spilled across our southern border under his watch, and got the situation back under control," Carter said.

Associated Press writers Paul Davenport, Michelle Price, Amanda Lee Myers, Martha Mendoza and Jonathan J. Cooper contributed to this report.