Loud blasts of sound from the sonar systems of Navy ships are killing and disorienting whales and other marine mammals and should be far more strictly limited, an environmental group argued in a federal lawsuit filed yesterday.

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

The group sued the Navy over its use of low-frequency sonar in 2002 and negotiated a settlement limiting its use. The new 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."

Lt. William Marks, a Navy spokesman, disputed NRDC's assertions, saying that "the Navy complies with the law. We recognize that active sonar testing and training must be accomplished in an environmentally sound manner."

The suit does not ask the Navy to stop using sonar -- which tracks submarines -- but to limit its use in testing and training, and to be more careful about where and when it gets turned on. Certain whale species are known to be especially sensitive to sonar noise, and their habitats and migration patterns are often known and can be avoided, the group said.

Scientists remain unsure how harmful sonar may be, but both the Navy and the National Oceanographic and Atmospheric Administration concluded that mid-frequency sonar caused a mass stranding of whales along a narrow channel in the Bahamas in 2000. There have since been similar strandings during Navy exercises off Hawaii, the Canary Islands, Washington state and North Carolina, but there is disagreement over whether sonar was responsible.

NRDC has also sued NOAA seeking information about the January 2005 stranding of 37 whales in North Carolina, near an area where the Navy wants to establish a sonar testing range. The group said NOAA supplied some documents but not the key necropsy results.

The agency initially said the inquiry would be completed by summer, but Donna Wieting, deputy director for protected resources for NOAA fisheries, said yesterday it is now expected to be finished by January. She said NOAA is conducting tests to determine what drove the animals ashore, and sonar noise is among the possible causes.

NOAA lead veterinarian Teri Rowles said the inquiry has been complicated by the fact that other "die offs" of individual marine mammals have occurred in the past year in the same region.

Marks said that the Navy cooperated with NOAA's inquiry and that its own review found that no Navy ships were using active sonar within 50 nautical miles during the four days before the animals were found.

In a statement, NRDC and four other groups that joined in the suit said mid-frequency sonar can emit continuous sound well above 235 decibels, roughly comparable to a Saturn V rocket at blastoff.