MONTERREY, Mexico – Drug hitmen shot dead six Mexican policemen on patrol in the marijuana-producing state of Sinaloa, the latest in a growing stream of attacks on police, the local attorney general's office said Friday.

A group of armed men blocked a busy road in Sinaloa's state capital Culiacan Thursday night and shot at the police with automatic weapons from two vehicles, a spokesman for the state attorney general said.

“We believe the killers were drug hitmen. The police were from different local and state units traveling together in the same vehicle,” he said.

The attack came hours after a lone gunman in Mexico City burst in on a regional police chief who headed operations against trafficking and contraband as he was having lunch at a restaurant on a busy street and shot him dead, along with a bodyguard.

Two other bodyguards with him were seriously wounded.

More than 560 police have been slain since President Felipe Calderón took office in December 2006 and launched a military crackdown on drug cartels, deploying some 25,000 troops and federal police across the country.

In May, Calderón sent thousands of troops to Sinaloa, a northern state that is home to Mexico's most-wanted man, Joaquin “Shorty” Guzman. Soldiers have seized stacks of cash, drugs and weapons, but the violence has continued unabated.

Across Mexico, drug killings and attacks on police have reached unprecedented levels as the army crackdown puts pressure on traffickers and intensifies battles over protection networks and smuggling corridors into the United States.

More than 1,600 people have died in drug violence so far this year and some 4,100 have been killed since December, 2006.

Last month one of Mexico's top federal police chiefs, Edgar Millan, was killed by hitmen linked to the Sinaloa drug cartel as he returned to his apartment in Mexico City.

Some corrupt police are killed for working for rival drug cartels, while honest cops are targeted to create fear among law enforcement agencies, anti-drug officials say.

Analysts who observe the drug trade say failure to go after cartels' finances amid too much focus on seizing drugs and weapons is undermining the military assault.

(Reporting by Robin Emmott; Editing by Catherine Bremer and Eric Walsh)