MEXICO CITY, June 1 (Xinhua) -- Armed men killed six police officers on Friday in the city of Salamanca, in Mexico's central state of Guanajuato, local authorities said.

Guanajuato prosecutor-general's office told Xinhua that the police officers were on a routine patrol in the southeast of the city when gunmen opened fire at them from inside a truck.

Five of the officers died on the spot while the sixth died of his wounds later in hospital, a government report said.

An investigation has been opened to identify the gunmen and the vehicle they were driving.

None of the police officers were armed as they were carrying out traffic duties and not public security, Guillermo Maldonado, a local official told the local newspaper, Correo.

"More must be done to try and stop these criminal acts," he was quoted as saying.

Since last year, Guanajuato, an industrial and agricultural state, has witnessed a wave of violence which the local government has blamed on criminal gangs fighting to control the sale of drugs and of fuel stolen from pipelines.