MEXICO CITY (Reuters) - One member of Mexico’s navy was killed and three were injured on Saturday when they came under fire while patrolling a section of state-run oil firm Pemex’s frequently plundered pipelines, the country’s naval secretary said.

Members of the navy were monitoring part of the Tuxpan-Azcapotzalco pipeline, which runs from the southeastern state of Veracruz to Mexico City, the navy secretary said in a statement, without saying exactly where the attack took place.

Mexican President Andres Manuel Lopez Obrador has vowed to crack down on the country’s rampant fuel theft, which cost Pemex an estimated $3 billion last year alone.

“Groups dedicated to fuel theft have increased the level of aggression against the staff of this institution,” the navy secretary said. “The navy secretary of Mexico rejects these actions and reaffirms its commitment to act firmly in defense of the peace of Mexico.”