Gunmen ambushed a police convoy in Mexico's western state of Sinaloa on Monday, sparking a shoot-out in which seven officers and four assailants were killed, officials said.
The convoy of two police vehicles was attacked as it drove from the coastal city of Los Mochis into the town of El Fuerte, said an officer in the El Fuerte police department.
Seven officers and four attackers were killed, he said. Officials did not know the motive of the attack, which bore the hallmarks of assaults carried out by drug cartels.
The Pacific state of Sinaloa is home to Mexico's oldest and wealthiest trafficking organization, the Sinaloa cartel, led by Mexico's most-wanted man - Joaquin "Shorty" Guzman.
The state has been a center of drug-related violence as the cartel battles rivals for control of billion-dollar narcotics trafficking routes to the United States.
There were other reports of violence elsewhere in Mexico on Monday.
In the northern city of Torreon, seven mutilated bodies were found along with notes typical of drug traffickers, said officials from attorney general's office for the state of Coahuila.
In the western state of Michoacan, police dug up six bodies in two separate pits, according to government-owned new agency Notimex.
More than 55,000 people have been killed in drug-related violence since outgoing President Felipe Calderon launched an army-led offensive against the cartels in 2006.
The government has blamed a turf war between the Sinaloa cartel and the brutal Zetas gang, founded by army deserters, for an upsurge in killings in recent months.
President-elect Enrique Pena Nieto, who will take office in December, has promised to rapidly reduce the homicide rate.