Thirteen bodies were found in an abandoned truck in eastern Mexico on Sunday, local media reported, as a turf war between drug cartels spreads far from the border with the United States.

The truck was found during a routine security patrol near the border between the eastern states of Veracruz, a major oil-producing region, and Tamaulipas, local media said, citing state officials.

Messages left at the site suggested the dead were killed in a rivalry between criminal gangs, local media said.

Violent drug cartels that have long menaced Mexico's northern border with the United States have moved into states like Veracruz as they battle rivals for control of drug routes and other criminality.

On Thursday, three American citizens were killed in Veracruz when gunmen attacked the bus in which they were travelling.

On Friday, the tortured bodies of ten people were found in Veracruz as a turf war between the Zetas gang and Gulf drug cartels intensifies.

In September, 35 bodies were dumped along a downtown highway in the Veracruz city of Boca del Rio.

More than 45,000 people have been killed in drug violence since President Felipe Calderon took office in December 2006. (Reporting by Patrick Rucker; editing by Todd Eastham)