MEXICO CITY (Reuters) - Mexican President Felipe Calderon vowed on Monday not to back down from the fight against powerful drug cartels who decapitated eight soldiers in the most serious blow to the army in a 2-year-old offensive.

Members of a drug gang are shown to the media at the Attorney General's office in the suburb of Escobedo, on the outskirts of Monterrey December 20, 2008. REUTERS/Tomas Bravo

Police found the beheaded and tortured bodies tied up in the city of Chilpancingo, about an hour north of Acapulco, during the weekend.

The heads were stuffed in a black plastic bag and tossed outside a shopping center with a note saying, “For every one of us you kill, we are going to kill 10,” Mexican media reported.

An ex-police commander, also without a head, was found with the soldiers.

The gruesome attack was the worst against the army since Calderon deployed some 45,000 troops to take on drug gangs after coming to office in 2006.

“We are committed to this fight with all of its consequences,” Calderon said at an event honoring a military hero. “We will not stand down and there will be no truce with enemies of the state,” he said.

Calderon’s assault against drug gangs has netted several major smugglers wanted in the United States, but violence in Mexico has worsened. More than 5,300 people have died this year, over twice as many as in 2007, as traffickers fight each other and the government over drug smuggling routes.

Washington, which has promised Mexico hundreds of millions of dollars in aid to buy equipment and provide security training, now sees Mexican cartels as its No. 1 drug threat.

It was not clear which faction was behind the beheadings. The main drug gangs are the Gulf cartel from northeastern Mexico and a federation of smugglers run out of the northwestern state of Sinaloa by Mexico’s most wanted man, Joaquin “Shorty” Guzman.

The violence threatens to scare away investors and hit Mexico’s economy, already shaky from the global financial crisis.

Mexican cartels are increasingly taking the place of the Colombian organizations who once ruled the international cocaine trade. Colombians have ceded many traditional trafficking routes to the United States to the Mexican gangs, preferring lower profile roles or focusing on Europe.

“There are no drug trafficking organizations left in Colombia that think they can go toe-to-toe with the nation-state; the cartels up in Mexico actually think that they can,” a senior Drug Enforcement Administration official based in Colombia told Reuters.

Calderon deployed the soldiers to fight organized crime in part because they are seen as less corrupt than police.

But military men from generals to foot soldiers have said they too are being offered thousands of dollars to turn a blind eye to shipments or call off anti-drugs operations.