Soldiers shield an emergency morgue erected to cope with scores of  bodies from mass graves discovered in the country's north.

Soldiers shield an emergency morgue erected to cope with scores of bodies from mass graves discovered in the country's north. Photo: AFP

Sixteen police officers in Mexico have been detained on suspicion of protecting a criminal gang that allegedly filled mass graves with 126 bodies.

Investigators searching shallow pits in San Fernando say they expect to find more bodies.

Officials from the northern border state of Tamaulipas said they uncovered 10 more bodies on Wednesday, bringing the total to 126 since the first were discovered last week.

Investigators have found so many bodies buried in mass graves in northern Mexico that they have begun moving the overflow to Mexico City morgues to help with identification.

The mass killings, mass graves and mass arrests of police have shaken the government of President Felipe Calderon, who has come under withering criticism in the congress and streets for his inability to stem the violence seen during his US-backed, military-led war against organised crime.

The government is sending additional troops to Tamaulipas, where locals have dubbed the main artery between the state capital, Ciudad Victoria, and Brownsville as ''the highway of death''.

Bus companies have cancelled travel, and the few Mexican journalists who have ventured into San Fernando arrived with military escorts.

Travellers are advised to drive a circuitous route hundreds of kilometres to the west through Monterrey to reach the border.

In February, two agents from the US Immigration and Customs Enforcement agency were ambushed on a major highway south of San Luis Potosi. Unarmed but driving an armoured sport-utility vehicle, one of the agents, Jaime Zapata, was killed by gunmen and the other was wounded.

Mexico's new Attorney-General, Marisela Morales, charged that the ruthless Zetas drug gang was responsible for the mass graves. The government announced $US1.25 million rewards for information leading to the capture of two Zeta chiefs who run criminal cells in and around San Fernando. Earlier, 17 suspects tied to the Zetas were detained. According to Mexican officials, the gang was kidnapping passengers off buses heading north.

Authorities believe Zetas may be kidnapping men to force them to work for the gang or holding them for ransom.

Last year, 72 migrants from Central and South America were victims of a mass killing in the same area. This group was headed towards the US border, and survivors said the migrants were killed for refusing to work for the cartel.

Seven major drug gangs are operating in Mexico, and more than 34,600 people have been killed since December 2006 in violence related to raging wars for control of smuggling routes and government efforts to stamp them out.

On Thursday authorities found the bodies of eight men, heaped in a pile and with signs of torture and gunshots to the head, in the western state of Michoacan, the stronghold of the La Familia drug cartel.

Meanwhile, three children were killed in the border town of Ciudad Juarez when their home was firebombed in other drug-related violence.