The suit, filed by the Natural Resources Defense Council in California, charges that the routine use of sonar in Navy training and testing is illegal under federal environmental law and is needlessly harmful.

The group sued the Navy over its use of low-frequency sonar in 2002 and succeeded in limiting its use under a negotiated settlement. Today's suit calls on the Navy to make changes to its far more extensive use of mid-frequency sonar as well.

"Military sonar needlessly threatens whole populations of whales and other marine animals," said Joel Reynolds, a senior attorney at NRDC. "In violation of our environmental laws, the Navy refuses to take basic precautions that could spare these majestic creatures. Now we're asking the courts to enforce those laws."

Navy spokesman Lt. William Marks disputed the NRDC claims, saying "The Navy complies with the law."

"We recognize that active sonar testing and training must be accomplished in an environmentally sound manner. The Navy has developed and implemented a comprehensive strategy for assessing the potential effects of its sonar use on marine mammals."

The lawsuit does not ask the Navy to stop using sonar — which is used to identify and track submarines from other countries — but rather to limit its use in testing and training, and to be more careful about where and when it gets turned on. Certain species of whale are known to be especially sensitive to sonar noise, and their habitats and migration patterns are often known.

While scientists remain unsure of how extensive harm from sonar may be, both the Navy and the National Oceanographic and Atmospheric Administration concluded that mid-frequency sonar did cause a mass stranding of whales along a narrow channel in the Bahamas in 2000. There have been subsequent reports of similar strandings during Navy exercises using sonar off Hawai'i, the Canary Islands, Washington state and North Carolina, but there is less agreement in those cases that the sonar caused the deaths.

The January 2005 stranding off North Carolina is the subject of an NRDC lawsuit against NOAA. The group requested information on that stranding — where 37 whales from three species were found on beaches as a sonar exercise took place in the region — and said some documents were supplied but the key necropsy results were not.

By law, NOAA investigates all mass strandings, and officials emphasized that they would do a comprehensive review of the North Carolina deaths because it caused public controversy. The strandings took place near an area where the Navy wants to establish an underwater sonar testing range.

The agency initially said the inquiry would be completed by summer, but today Donna Wieting, deputy director for protected resources for NOAA fisheries, said that it is now expected to be finished by January. She said NOAA is conducting a broad range of tests to determine what brought the animals to shore and noise from sonar remains one of a number of possible causes.

NOAA lead veterinarian Teri Rowles said the inquiry into the North Carolina mass stranding has been complicated by the fact that a number of other "die-offs" of individual marine mammals have occurred within the past year in the region.

Navy spokesman Marks said the Navy cooperated with NOAA regarding the North Carolina stranding and its review found that no Navy ships were using active sonar within 50 nautical miles of the stranding on the day the animals were found or the four days before.

In a statement, NRDC says mid-frequency sonar can emit continuous sound well above 235 decibels, an intensity roughly comparable to a Saturn V rocket at blastoff. That volume is made even more extreme by the fact that marine mammals have sensitive hearing, the group says. In its suit, the group reports that the scientific committee of the International Whaling Commission has concluded that the sonar is harmful to at least one species of whale — the deep-diving beaked whale — and possibly more.

"Whales exposed to high-intensity mid-frequency sonar have repeatedly stranded and died on beaches around the world; some bleeding from the eyes and ears, with severe lesions in their organ tissue," NRDC said in its statement. "At lower intensities, sonar can interfere with the ability of marine mammals to navigate, avoid predators, find food, care for their young, and, ultimately, to survive."

Marks disputed the NRDC description of how loud sonar can be — saying that noise behaves differently in water than in air — but he added that the Navy goes to "great lengths" to avoid marine mammals during exercises because of the possible harm.