MONTERREY, Mexico, May 23 (Reuters) - Suspected drug hitmen dumped four human heads in ice chests in northern Mexico on Friday in a gruesome murder of rivals, a state attorney general's office said.

"They were four men, beheaded by groups linked to drug trafficking. We believe the victims were drug gang members themselves," a spokeswoman said.

The heads were placed in four separate ice chests on Friday morning on a highway on the edge of the mining city of Durango and were found by police patrolling in the area.

Durango state is close to the Pacific state of Sinaloa, home to major traffickers including Mexico's most wanted man, Joaquin "El Chapo" (Shorty) Guzman.

Some 1,300 people have been killed in drug violence across Mexico this year and more than 2,500 died in 2007 as gangs vie for smuggling routes into the United States.

Drug cartels frequently torture and behead rivals.

In a separate incident in Ciudad Juarez across from El Paso, Texas, police found five bodies wrapped in blankets and dumped in an empty lot on one of the city's main avenues on Friday, the Chihuahua state attorney general's office said.

Two of the bodies were beheaded and drug gangs left a warning message reading: "This is what happens to stupid traitors who make the mistake of siding with El Chapo Guzman."

President Felipe Calderon has sent some 25,000 troops and federal police to quell the drug war across the country since taking office in December 2006, making big narcotics seizures and arresting drug kingpins.

But violence continues to escalate as drug gangs fight each other and target troops and police officers. (Reporting by Robin Emmott and Ignacio Alvarado; Editing by Eric Beech)

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