MEXICO CITY, May 5 (Reuters) - Gunmen killed a police chief in the southern Mexican state of Chiapas on Saturday, in the latest apparent sign that President Felipe Calderon's campaign against drug gangs has failed to contain violence.

A group of men shot state police chief Manuel Cordova as he traveled in a truck in the city of Tapachula near the border with Guatemala, state authorities said.

Calderon has sent thousands of troops to states on the U.S. border and other regions to clamp down on narcotics cartels that are behind a wave of drug-related brutality.

Despite the campaign, execution-style killings and other grizzly violence have become increasingly common and spread to once peaceful states.

Gangs, flush with cash from the drug trade, regularly outgun local police by using assault rifles and grenade launchers, and can often easily buy the cooperation of poorly paid cops.

On Thursday, gunmen attacked the police chief of Caribbean beach resort Cancun, killing one of his bodyguards, although he escaped unharmed.

Masked gunmen killed five soldiers in a shootout in the western state of Michoacan earlier in the week.

Drug-related deaths in Mexico number more than 700 so far this year. Narcotics-related violence left 2,000 people dead in 2006.

Drug gangs in Mexico occasionally assassinate senior local cops and it is sometimes unclear whether they have been targeted because of involvement with organized criminals or in retribution for trying to catch them.

Calderon told soldiers on Saturday their fight against organized criminals would continue.

"We will double our offensive against the enemy because more energy should be the response to greater violence," he said in a speech commemorating Cinco de Mayo holiday.

(Addidional reporting by Monica Medel)

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