Troops drove through the rough suburb of Santa Catarina, where assailants with Kalashnikov rifles gunned down a traffic police director on Monday, as well as the wealthy suburb of San Pedro Garza García and the center of Monterrey.

On Monday, 150 state police officers in the metro area of 3.2 million temporarily walked off their jobs to protest being forced to work overtime because of the deaths, arrests and resignations of their colleagues.

Federal officials recently detained 18 officers in the state, Nuevo León, for allegedly working with drug cartels.

Nuevo León Gov. Jose Natividad González said many of the municipal and state officers killed were involved in organized crime. "We are going through difficult moments. The security forces have been victims of the intense pressures of both corruption and threats," he said.

President Calderón said Tuesday he will try to provide resources and pay incentives for police who are on the front lines of his offensive against drug cartels.

Nuevo León state police officers earn about US$760 a month for the life-threatening job.

State police chief Antonio Garza said 91 officers were being trained to make up for troop losses and he said police were enrolled in an anti-stress program to cope with the job conditions.

The Defense Secretariat said it could not disclose how many soldiers were mobilized or how long they would remain in Monterrey, citing security reasons.