The Mexican army has seized about 840 metric tons of chemicals used for making methamphetamines, in one of the largest such seizures in Mexico to date.

Mexico's Defense Department says soldiers found sacks of the materials piled high in a warehouse in the north-central state of Queretaro. One security expert says the size of the haul, which was photographed, indicates drugs were being produced on an industrial scale.

In 2006, President Felipe Calderon launched an army-led crackdown on Mexico's drug cartels, which often smuggle their product across the border into the United States.

Since then, more than 41,000 people have died in violence linked to Mexico's drug gangs.