(CNN) Drug cartels illegally growing marijuana on federal public lands in California are getting bolder and using banned, toxic pesticides, officials said Tuesday.

The secret marijuana sites are organized, funded and operated by Mexican cartels, said US Attorney McGregor Scott. The issue is particularly stark in California, which has more than 16 million acres of national forest lands, including Yosemite, Kings Canyon and Lassen Volcanic National Park.

"This is not a bunch of people growing marijuana in the woods," Scott said in a news conference Tuesday. "This is people who are using chemicals and causing manifest environmental damage to a crown jewel of the United States -- our federal public lands."

The illegal growers divert the water supply, use toxic chemicals banned in the US and clear native plants and trees, in order to grow marijuana plants on what were pristine federal lands.

The issue of illegal marijuana farms on public lands has persisted in California for years, but Maj. Gen. David Baldwin of the California National Guard said they had seen a "tremendous uptick and increase in the amount of environmental damage and degradation that's done by the growers, that have become bolder and bolder to take the public lands."

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