SACRAMENTO  (AP) — Law enforcement crackdowns on illegal marijuana groves on California's public lands appear to be pushing some growers into other states, federal authorities said Tuesday, prompting a multi-agency effort across the West this summer.

There has been a decrease in the number of marijuana plants seized in California in recent years, though the state still accounts for the bulk of the illegal pot harvest. There has been an increase in some neighboring states, officials said, so they are responding with what they're calling Operation Mountain Sweep in seven states west of the Rockies.

"Marijuana grown in this state supplies the nation," Benjamin Wagner, the U.S. attorney in Sacramento, said at a news conference. "Huge amounts of marijuana growing here are flowing to the East, to other states in the East."

Yet years of arrests and prosecutions in California are having an effect, he said, reducing the number of plants found on public lands each year in the state. More than 5 million plants were seized on public land in 2010 in California, but that was down by 3 million plants from 2009. Fewer than 2.5 million plants were destroyed last year, and this year is likely to see another reduction, Wagner said.

"Because of the pressure that we're putting on these organizations here in California, there may be an uptick in some other states," Wagner said. Results are mixed, he said, but Nevada is among states seeing more illicit grows there.

California still accounted for 83 percent of the marijuana seized in the western region, however.

Since the operation began July 1, local, state and federal agents have seized 734,000 marijuana plants in California — six times more than all of the other western states combined.

They destroyed nearly 48,000 plants in Idaho, 25,000 in Arizona, 19,000 in Washington, 18,000 in Nevada, nearly 10,000 in Utah, but just 300 in Oregon, as of last week. The operation will continue through the end of the month.

The federal Drug Enforcement Administration said the value of the nearly 500,000 plants seized on public land exceeds $1 billion, not counting the additional plants found on neighboring privately owned property.

The illicit plantations can devastate public land and endanger those who are hiking, hunting or fishing on public land, officials said.