WASHINGTON — The Navy is moving ahead with plans to build a 500-square-mile sonar training range off the coast of North Carolina, officials said last week, a project that has sparked fierce opposition from environmentalists who say some of the world's most endangered whales and sea turtles pass through the area.

Planning for the $99 million range has been under way for almost 10 years, but environmental challenges and concern that the sound waves from sonar may harm protected marine mammals have held up the process. The Navy published its draft environmental-impact statement Friday and will begin a series of public hearings on the proposal next month.

The proposed site, about 50 miles off North Carolina, was selected to provide the Atlantic fleet with training in the use of sonar in coastal areas, where the Navy believes the greatest submarine threats exist. The global spread of quiet and relatively low-cost diesel submarines has alarmed the Navy and convinced officials that its sailors need more training in detecting hostile subs in canyons and ocean beds closer to shore.

But marine-mammal researchers and environmentalists have grown increasingly alarmed over the Navy's plans and the potentially damaging effects of active sonar — which sends out very loud blasts of underwater sound.

Whales and other marine mammals have very sensitive hearing, and a growing body of research has shown that sonar can disorient and sometimes kill them. The Natural Resources Defense Council, an environmentalist group, sued the Navy last week over its use of midfrequency sonar, the type that would be deployed at the new sonar range. The group said the sonar threatened endangered animals, in violation of several federal environmental laws.

Marine-mammal advocates say they see the proposal for an East Coast sonar range as a long-feared "test case" of increased Navy assertiveness — especially since one of the most endangered and highly protected whales on Earth migrates through the region.