A prominent preservation group says 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.

The nonprofit National Trust for Historic Preservation says in a report to be released today in Denver that only about 1,900 of 325,000 Forest Service sites identified as historically or culturally significant are on the National Register of Historic Places.

And about 80 percent of the 193 million acres the agency manages in 44 states and Puerto Rico haven't been surveyed for such sites.

The Washington-based trust says 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.

Forest Service officials say they welcome the recommendations and have been considering changes to policy and manuals to emphasize the importance of cultural resources.