MONTERREY, Mexico, Feb 27 (Reuters) - Gunmen have shot dead a Mexican police chief in a daylight ambush outside his office in the border town of Agua Prieta, a strategic U.S entry point for illegal narcotics, local authorities said on Tuesday.

Police chief Ramon Tacho was shot by a group of men with assault rifles driving two four-wheel drive vehicles on Monday afternoon, a spokesman for Agua Prieta's town hall said.

"Chief Tacho was shot several times and died in hospital. It was the bullet in his right lung that was fatal," said the spokesman, who declined to be named.

Agua Prieta, in Mexico's Sonora state, lies across the border from Douglas, Arizona, and is home to assembly-for-export plants, known as maquiladoras. It is also a vital crossing point for South American cocaine.

Some 2,000 people were killed in drug violence in Mexico last year. Since taking office in December, President Felipe Calderon has sent thousands of troops across Mexico to try to weaken drug cartels.

But the United States and human rights groups have warned of a backlash and dozens of active and retired police officers have been killed across northern Mexico so far this year.

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