The coalition says that mid-frequency, high intensity sonar systems used on 60 percent of Navy ships and submarines to detect enemy submarines interferes with the ability of marine mammals to navigate, find food, avoid predators and communicate with each other.

"Without reasonable limits, the proliferation of high intensity sonar will cause excruciating pain, injury and death for an increasing number of marine mammals," said Frederick O'Regan, president of the International Fund for Animal Welfare.

A Navy spokesman in Washington D.C. said the letter would be "carefully reviewed and considered" and noted that the Navy already has several programs aimed at protecting marine mammals.

The letter was prompted by what the coalition called a stampede of about 200 melon-headed whales during a U.S.-Japanese naval training exercise off the coast of the Hawaiian island of Kauai two weeks ago.

The pod of normally deep-water whales crowded into shallow waters near the shore in such chaos that one of the whales stranded and died. The warships shut off their sonar on learning of the stampede but the coalition said the exact sequence of events was unclear.

The letter cited 10 cases of mass stranding and whale deaths associated with mid-frequency sonar in the last nine years in places ranging from Greece to the Canary Islands.

Citing the journal Nature, the coalition said that intense sonar blasts can give marine mammals decompression sickness similar to "the bends" sometimes experienced by surfacing divers. Post-mortem examinations on some whales exposed to sonar showed hemorrhaging around the ears and the larynx.

Last year the Navy agreed to scale back the use of a different kind of sonar system using low-frequency waves after losing a lawsuit brought by the coalition under endangered species and marine mammal protection legislation.

The coalition said the Navy's use of mid-frequency sonar violated the same laws but on a much larger scale. "We'd rather not resort to litigation so we are once again asking the Navy to sit down to discuss this in a spirit of co-operation," said NRDC lawyer Joel Reynolds.