DENVER — The U.S. Forest Service lacks a clear legal mandate and the financial ability to protect thousands of historic sites and buildings on national forest lands from development, vandalism and other threats, a prominent preservation group says.

The nonprofit National Trust for Historic Preservation released a report Thursday saying only 1,936 of 325,000 Forest Service sites identified as historically or culturally significant are on the National Register of Historic Places.

"We think that's just the tip of the iceberg. We think there could be as many as 2 million sites," trust president Richard Moe said in a telephone interview with The Associated Press.

At-risk treasures include American Indian pueblos and sacred sites, petroglyphs, Revolutionary and Civil War battlegrounds, trails used by the Lewis and Clark expedition and Forest Service lookout towers.

About 80 percent of the 193 million acres the agency manages in 44 states and Puerto Rico haven't been surveyed for such sites, according to the trust based in Washington, D.C.

"The National Forest System: Cultural Resources at Risk" says the Forest Service, unlike other federal land management agencies, has no statute that specifically mandates historic or archaeological preservation as part of its mission.

Another issue is funding. Less than 1 percent of the Forest Service's $4.4 billion budget goes to heritage resource programs, according to the report. Nearly half its budget is spent on fires, including fire suppression and decreasing wildfire risk.

Threats to historic and cultural sites include off-road vehicle use, oil and gas development in the West, livestock grazing, logging and a resurgence in uranium, gold and other hard-rock mining, Moe said.

Forest Service personnel recently discovered a 10-foot-tall painting of the cartoon character SpongeBob SquarePants on the chimney of a historic cabin in central Colorado's Pike National Forest.

"Vandalism is increasing because there aren't enough forest rangers to protect these places," Moe said.

In Missouri, the trust has placed 70 historic buildings slated for demolition in the Mark Twain National Forest on its most endangered places list. The trust says the forest's master plan gives priority to new construction over preserving historic structures.

"The preservation of these national treasures should be a top priority of our elected officials and it's not," Moe said.

Joel Holtrop, deputy chief of the National Forest System, said the Forest Service is serious about its responsibilities under the National Historic Preservation Act to protect cultural resources and is proud of the job it does.