(Updates death count for 2008, adds attorney general quote)

MONTERREY, Mexico, May 23 (Reuters) - Suspected drug hit men dumped four human heads in ice chests in northern Mexico on Friday in a gruesome killing 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 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.

Gangland killings have surged again in Mexico in recent weeks and organized crime murders across the country total 1,378 this year, Mexico's attorney general said on Friday, up 47 percent from this time last year.

Drug violence killed more than 2,500 people in 2007 as rival gangs fought over smuggling routes to the United States.

Torturing or beheading rivals is common in drug cartel killings.

In an 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 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 is still rising as drug gangs fight each other and target troops and police officers.

Mexican Attorney General Eduardo Medina Mora told radio the latest surge in bloodshed, most marked in the northern states of Chihuahua, Sinaloa and Baja California, reflected the army operation squeezing cartel operations.

"There are more disputes between them because the cake has been made smaller," he said. (Reporting by Robin Emmott, Ignacio Alvarado and Miguel Angel Gutierrez; Editing by Peter Cooney)

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