Groups suing to block a new federal rule say it could result in the killing of as many as 600 of the Northern Rockies' 1,500 wolves.

But state and federal officials say the actual number will be much smaller.

Seven environmental groups announced Monday they had filed an injunction request in U.S. District Court in Missoula, Mont., to block the rule. It would give the Wyoming Game and Fish Department and other state wildlife agencies the authority to take out wolves if it's shown they are one of several major factors in keeping down elk herds.

Earthjustice attorney Doug Honnold, who filed the lawsuit on behalf of the environmental groups, pointed out that Wyoming and Idaho in particular wanted the new rule, to gain more control over wolves in the event there's a delay in removing the animal from federal Endangered Species Act protection. Montana officials have said they are not likely to take advantage of it.

"Hundreds of wolves could be killed because of the states' efforts to reduce wolves in areas where there are elk, their native prey, Honnold said. "That's a question of whether wolves belong on the western landscape or not."

Federal officials said there are two elk units in Idaho that might qualify under the new rules and three or four in Wyoming. Before wolves are killed, there would have to be a public comment period, peer review and approval from the U.S. Fish and Wildlife Service.

Steve Nadeau, wolf biologist with the Idaho Department of Fish and Game, said the groups that filed the lawsuit were "playing the fear card" to stop state management of the predator.

"The state has promised wolves are here to stay," he said. "It's time to start managing them as any other big game animals."

The Fish and Wildlife Service is preparing to remove wolves from the endangered species list next month. Under delisting, all three states could set public hunting and trapping seasons for wolves. If the number of wolves dropped below 300, they would be returned to the endangered list.

The rule giving states more leeway to kill wolves before delisting is set to go into effect in 30 days but could be short-lived. The proposal to remove wolves from the endangered species list is expected to be finalized in the coming weeks. Delisting would nullify the rule. Legal challenges to delisting, though, could keep it alive.

The plaintiffs in the lawsuit filed Monday are Defenders of Wildlife, the Natural Resources Defense Council, the Sierra Club, the Center for Biological Diversity, the Humane Society of the United States, the Jackson Hole Conservation Alliance, and Friends of the Clearwater.