About 37,000 people have been killed in drug violence since the government launched a crackdown in 2006 [AFP]

Mexican authorities say they have deployed an extra 1,800 federal agents in their battle with drug cartels, after at least 41 people were killed over the weekend, including 10 who were beheaded.

The agents were sent to Michoacan state to take part in a battle there mainly with the Knights Templars, a splinter group of the La Familia cartel.

The reinforcements were backed by 170 vehicles, 15 ambulances and 4 MI and Black Hawk helicopters, the Public Safety office announced on Saturday.

Alejandro Poire, a national security council spokesman, called it a "reinforcement operation faced with the possibility of greater mobilisation by organised crime groups".

"They are waging an absurd war, to the death, for control of criminal turf and drug trafficking routes to the United States," he said.

The federal government already has about 50,000 army troops and thousands of federal police in its fight against drug cartels.

It blames the groups for most of the 37,000 people killed since it started a military offensive against the cartels in December 2006.

Many of the dead have been killed by the cartels as a form of intimidation or for refusing to co-operate with drug traffickers.

Headless bodies

Officers in the northern city of Torreon said on Saturday that they had found the headless bodies of seven men and three women in the back of an abandoned pick-up vehicle.

Guillermo Flores, the city's police chief, told the AFP news agency that only one head, belonging to a woman, was found on site, and that the killers had placed it on the lorry's hood.

The vehicle was parked on a road that goes around Torreon, a city of about 650,000, where two major roads heading north to the US converge.

The victims had apparently been executed "several days ago" in disparate locations, and their bodies were piled up in the abandoned pick-up vehicle "in an attempt to sow terror among the citizens of Torreon", the city municipal police said in a statement.

Police have not said if they have suspects.

Coahuila state, where Torreon is located, is a battleground for two powerful Mexican drug cartels: the Zetas, founded by former Mexican special forces soldiers, and the Pacific cartel, headed by Joaquin "Chapo" [Shorty] Guzman.

The decapitated bodies were discovered hours after 20 people were killed late on Friday when assailants attacked a local bar in Monterrey, located in the north and the country's second richest city.

The attackers, who arrived in two pick-up vehicles and a car, stormed into a bar in Monterrey's busy nightlife district and opened fire on the patrons.

Separately, 11 people were shot dead on Friday afternoon in Chalco, a town on the outskirts of Mexico City.

Source: Agencies