MONTERREY, Mexico - Faced with ever-worsening drug cartel violence, Mexican President Felipe Calderon is turning to elite navy troops to help reignite his floundering army-led drug war.

The navy scored the biggest victory yet of Calderon's three-year struggle to stem drug violence when its special forces killed a top drug lord this week, showing how it is breaking out of its traditional role and moving inland.

Naval troops shot dead Arturo Beltran Leyva, aka The Boss of Bosses, in a raid at a luxury apartment complex in a city near the capital. The heavily armed troops in ski masks pinpointed Beltran Leyva's location, descended from helicopters to surround him and opened fire in the operation known only to a handful of government officials.

The strike followed a string of successful navy operations across Mexico, including the killing of a top Gulf cartel hitman in the northern city of Monterrey this month.

Analysts say the navy's special training and small size make it more effective and less at risk of leaking operational information on important raids than the army or police.

By bringing naval troops into the drug fight, however, Calderon risks exposing them to the corrupting power of drug gangs who have long infiltrated the police and are now doing the same with parts of the army.

A 49,000-strong army deployment is still the driving force behind Calderon's drug war, but the army faces accusations of rights abuses and that some soldiers and elite police units are in the pay of drug cartels.

The army assault has also stoked turf wars between rival cartels, worrying foreign investors and Washington as killings soared above 7,000 people this year, taking the death toll over three years to above 16,000.

The navy is coming in at a time when the army risks becoming discredited in terms of human rights and Calderon's strategy was running out of ideas, said security expert Pedro de la Cruz at the National Autonomous University of Mexico.

But you can't get away from the fact that this is only a temporary boost and that police and justice reform is still key to resolving the problem of organized crime.

RIFT IN THE ARMED FORCES?

In their combat gear, navy troops look virtually identical to Mexican soldiers, but they fiercely guard their independence from the army and keep a distance from the often corrupt police forces the government is trying to reform.

We work alone, we don't trust the police. We are committed to this fight, a navy captain in Tijuana on the U.S. border told Reuters. He declined to be named for his safety.

In northern border cities like Tijuana and Ciudad Juarez, soldiers have lost local support, as they clumsily burst into homes and bars looking for drug suspects and harass even school children at military checkpoints. Some troops have been accused of torture, but the army denies any wrongdoing.

The navy, which guards ports and coasts, has a much lower profile in Mexico and until recently had played a marginal role in the drug war.

Still, army support remains crucial and if generals perceive they are being shut out of major operations or only playing a back-up role, it could undermine morale.

The navy's success so far also relies on keeping its intelligence to itself for fear of leaks. That strategy could limit its impact given its relatively small size of 15,000 troops active in the drug war across Mexico, while Calderon needs unity throughout the armed forces.

You've got to question whether the armed forces are coordinating between themselves or if we are seeing a situation of distrust, which would seriously damage the government's efforts, said independent security consultant Julian Gudino.

(Additional reporting by Lizbeth Diaz in Tijuana; editing by Catherine Bremer and Mohammad Zargham)

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