MEXICO CITY - 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.

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

Their heads were stuffed in a black plastic bag and tossed outside a city shopping centre next to a note saying, 'For every one of us you kill, we are going to kill 10...,' Mexican media reported.

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

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

'We are committed to this fight with all of its consequences,' Mr Calderon said at an event honouring a Mexican military hero who fought the 19th century war of independence against Spain.

'We will not stand down and there will be no truce with enemies of the state,' he said.

Mr Calderon's offensive has helped fuel a major increase in drug violence. More than 5,300 people have died so far this year, over twice as many as in 2007, according to the attorney general's office, as rival traffickers fight each other and the government.

The army said the attacks were a sign drug traffickers were weakening under high-profile captures of major cartel leaders and massive drug hauls in recent months.

Mr Calderon deployed the soldiers in part because they are seen as less corrupt than police, who have been widely found to accept bribes from drug lords, up through the highest ranks.

But military men from generals to foot soldiers in the violent border state of Baja California have said they too are being offered up to hundreds of thousands of dollars to turn a blind eye to shipments or call off anti-drugs operations. -- REUTERS