The reputed head of Mexico's Sinaloa drug cartel is threatening a more aggressive stance against competitors and law enforcement north of the border, instructing associates to use deadly force, if needed, to protect increasingly contested trafficking operations, authorities said.

Such a move by Joaquin "El Chapo" Guzman, Mexico's most-wanted fugitive, would mark a turn from the cartel's previous position of largely avoiding violent confrontations in the U.S. -- either with law enforcement officers or rival traffickers.

Police and federal agents in Arizona said they had recently received at least two law enforcement alerts focused on Guzman's reported orders that his smugglers should "use their weapons to defend their loads at all costs."

Guzman is thought to have delivered the message personally in early March, during a three-day gathering of his associates in Sonoita, a Mexican town a few miles south of the Arizona border, according to confidential U.S. intelligence bulletins sent to several state and federal law enforcement officials, who discussed them on the condition of anonymity.

The Sonoita meeting is considered one of several signs that Guzman is becoming more brazen even in the face of a Mexican government crackdown on his activities and continued turf rivalries with other traffickers.

Information from informants, wiretaps and other sources have prompted a flurry of warnings to authorities in U.S. border states, instructing them to use extreme caution when confronting people suspected of smuggling drugs and illegal immigrants from Mexico or ferrying weapons and cash south from the United States, officials familiar with those warnings said.

Some U.S. intelligence officials suggested Guzman was on the defensive because of enforcement efforts on both sides of the border and because he can no longer afford to ditch valuable cargoes when challenged by rivals or authorities.

Michele Leonhart, acting administrator of the Drug Enforcement Administration, said Mexican smugglers were also under pressure because their Colombian partners were no longer extending them credit. "There's a need to get the cash back itself quicker and faster," Leonhart said.

U.S. authorities say Guzman has become increasingly intent on gaining dominance over smuggling routes in Mexico and the United States. To do so, they say, he has escalated his assault on some rival smugglers while forging alliances with others.

"Chapo is at the forefront of the efforts to control the routes into the United States," said Thomas M. Harrigan, the chief of operations for the DEA.

He said virtually all of the violence remained in Mexico, but U.S. authorities were alarmed that attacks on police, soldiers, government officials, journalists and other potential opponents had intensified near the border.

How much risk that poses to U.S. authorities "depends on how desperate the cartels become to move their cargo in the U.S.," said Dan Wells, commander of the Arizona Department of Public Safety's Intelligence Bureau.

So far, the contrast has been stark -- near-daily violence in Mexican border towns with relative tranquillity on the U.S. side, according to data and interviews with law enforcement officials in the region.

For example, Ciudad Juarez had 100 times as many homicides in the 14 months ending in February as neighboring El Paso, which is roughly half its size. In 2008, Nogales in Mexico's Sonora state had 40 times as many homicides as Nogales, Ariz., which is roughly one-ninth as populous.

Deeper into the United States, narcotics agents say they have seen little evidence of spillover from Mexican drug war violence beyond an increase in ransom kidnappings related to collection of drug debts.

But near the Mexico-Arizona border, Robert W. Gilbert, chief patrol agent for the U.S. Customs and Border Protection's Tucson sector, said confrontations between law enforcement and suspected traffickers -- and among traffickers themselves -- had grown more violent.

A shootout occurred several weeks ago when one group allegedly tried to hijack another's load of drugs on one of the main roads leading north to Phoenix. Two of the suspected traffickers were wounded.

"Times have changed," Gilbert said. "The tactics, the aggressiveness. We're victims of our own success." Now, he said, "they'll fight us."

An internal report from the agency, obtained by the watchdog group Judicial Watch, appears to support Gilbert's assessment. It shows reported weapons-related assaults against border officers rose 24% last fiscal year, compared with 2007, and assaults involving vehicles rose 7% in the same period.