A federal timber sale in southwestern Montana should be stopped because it would violate a Forest Service plan for wildlife and trees, environmental groups say in a lawsuit against the agency and its regional forester, Tom Tidwell.

The suit filed Monday in U.S. District Court in Missoula by the Alliance for the Wild Rockies and the Native Ecosystems Council challenges plans for a sale in the Fleecer Mountains about 10 miles southwest of Butte.

The groups say the project calls for clearcutting 133 acres, including some old-growth trees, in violation of a forest plan.

Forest Service spokesman Ed Nesselroad said Tuesday the agency had no comment.

The project in the Beaverhead-Deerlodge National Forest would remove dead trees, or ‘‘snags,’’ important to birds and wildlife such as elk, said Michael Garrity, executive director of the Alliance for the Wild Rockies.

Birds nest in the dead trees’ cavities, and those trees provide cover for elk, Garrity said.

‘‘The forest plan requires the Forest Service to designate replacement snags for wildlife, but instead the Forest Service is clearcutting an area where they admit there probably are not enough snags,’’ said Sara Johnson, Native Ecosystems Council director and a former wildlife biologist for the Gallatin National Forest.

Forest Service studies found that clearcutting would harm species such as the hairy woodpecker and the pine martin, Johnson said.