Border areas where rival drug cartels are battling for control of key routes into the United States are among the worst hit in escalating violence across Mexico this year in which almost 3,500 have died, according to local media.

Attacks have increased despite a government crackdown on drug-related violence including the deployment of 36,000 troops across the country.

Police in Tijuana found five bodies with their hands and feet tied and heads wrapped in tape on Friday, as well as two headless bodies and one other body, making a total of 42 dead in one week.

Gangland-style murders have escalated this week in the city across the border from San Diego, including the discovery of eight bodies Thursday, and a pile of 12 bodies found in a nearby area on Monday, local officials said.

In Chihuahua state a former district attorney and four local police were killed, including a woman, out of a total of 19 dead, officials said Friday. In state capital Chihuahua, several armed men fired from a car on the former district attorney late Thursday, state authorities said.

"On seeing his attackers he tried to run away, but they followed him and shot him several times in the back," a state official said.