The camps are designed to train recruits, which range from Mexican military deserters to American youths, to carry out killings and other cartel assignments on both sides of the U.S-Mexico border, The Dallas Morning News reported Sunday.

"Traffickers go to great lengths to prepare themselves for battle," a senior U.S. anti-narcotics official told the newspaper, speaking on condition of anonymity. "Part of that preparation is live firing ranges and combat training courses. ... And that's not something that we have seen before."

The newspaper reported that many of the training camps are used temporarily then abandoned, while others are permanent, fortified facilities.

In Texas, Webb County Sheriff Rick Flores said he and other law enforcement officials are struggling to "secure" the border amid escalating violence among rival drug cartels.

"We know through intelligence sources that narco-traffickers invest money in Mexican nationals and U.S. citizens in training camps to instruct them in the black art of assassination and terror," he said. "It's even more shocking to hear that they even have mobile training sites because they take loads of money to set up."