Organized crime gangs equipped with automatic weapons and tractor trailers are branching out into raids on huge grain silos, in a sign of growing lawlessness in parts of Mexico's north.

Organized crime gangs equipped with automatic weapons and tractor trailers are branching out into raids on huge grain silos, in a sign of growing lawlessness in parts of Mexico's north.

Attacks on warehouses and cargo trucks have multiplied into a near-weekly affair in the northwestern state of Sinaloa, where one of the worst cold snaps in decades wiped out corn and vegetable plots last month, pushing up prices of the remaining harvest and making it more attractive to thieves.

The unusual crime wave in major agricultural exporting states is a new headache for the Mexican government struggling to maintain the country's image as a top emerging market.

Mexico's national warehousing association AAGEDE said the spike in thefts began a year or two ago, but its members are only recently coming forward and many are still too scared to report details on the number or scale of the incidents.

Jose Jimenez, director of Mexican storage company ALMER, told of one robbery last year in a tiny town in the central state of Zacatecas where an armed commando emptied a warehouse of 900 tonnes of beans, worth around $750,000, loading up 30 trucks over the course of an entire day.