MONTERREY, Mexico, May 12 (Reuters) - Drug cartel hitmen dumped a severed head outside a military base in the Mexican port of Veracruz to warn newly arrived troops of more violence in an escalating war on drug traffickers, authorities said on Saturday.

"We found the head of a man on Friday night with a message warning the army not to mess with the drug cartels, to expect a fierce battle," said an official at the Veracruz state attorney general's office who declined to be named.

Mexican President Felipe Calderon sent hundreds of soldiers and federal police to Veracruz on the Gulf of Mexico on Friday night after gunmen killed four Mexican policemen who worked as bodyguards for the family of a senior politician on Thursday.

Authorities suspect the powerful Gulf cartel or a rival break-away gang, "Gente Nueva" (New People), were behind the beheading and its message.

Drug violence in Mexico has spiraled since the start of the year as the Mexican army and police try to end a war between a coalition of gangs on the Pacific coast and the Gulf cartel -- based just south of Texas -- for dominance of smuggling routes to the United States.

At least 800 people have been killed this year.

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