A dozen Mexican soldiers were arrested on suspicion of working with the violent Gulf Cartel, the Mexican army said on Thursday, a blow to President Felipe Calderon's military-backed campaign against drug gangs.

The troops are accused of collaborating with four municipal policemen in the central state of Aguascalientes who provided protection for Gulf cartel capos, the army said in a statement.

The arrests come as Calderon sent thousands more troops to the violent border city of Ciudad Juarez in an attempt to curb spiraling drug violence that killed more than 6,000 people last year.

Calderon deployed the army to fight organized crime since taking office in 2006 partly because soldiers have traditionally been seen as less corrupt than police.

But several recent high-profile arrests -- including a presidential guardsmen who allegedly received $100,000 a month to track Calderon for drug traffickers -- reveal infiltration in the highest levels of Mexico's security forces.

The Gulf cartel is fighting a turf war for control of smuggling routes with its main rival the Sinaloa federation, led by Mexico's most wanted man Joaquin "Shorty" Guzman.

The Gulf cartel's feared hitmen known as the Zetas, infamous for torturing and beheading their enemies, were founded by a group of military deserters.