Sheriff's Lt. Robert Tersigni said law-enforcement agencies began uprooting plants early Wednesday morning on information gathered from interviews and following up leads.

Tersigni said officers from several local, state and federal agencies began eradicating plants in four areas early Wednesday.

"So far we've cut between 10,000 and 20,000 plants," he said, noting the total amount could reach 50,000.

"This could be the biggest eradication effort in Utah's history," Tersigni said.

The plants are growing across several miles in the rugged mountains north of St. George, said county spokesman Dean Cox.

Once cut, the pot plants will be flown to the Pine Valley fire station where they will be destroyed.

No arrests have been made, but the area is under observation.

The operation is the third large-scale marijuana eradication effort this summer in southwest Utah.

Last week, about 3,000 robust plants were found growing in a canyon south of Cedar City in Iron County.

And last September, Washington County authorities found 1,000 marijuana plants -- some reportedly as tall as 15 feet -- worth as much as $5 million growing in hills west of New Harmony and the Dixie National Forest.

Agencies taking part in Wednesday's efforts included the federal Drug Enforcement Agency, Washington County Sheriff's Office and Search and Rescue, St. George police, Hurricane police, Kane County Sheriff's Office, Utah Department of Public Safety, Utah National Guard, U.S. Forest Service, Immigrations and Customs Enforcement and the Bureau of Land Management.