HONOLULU -- The Navy will use sonar as planned next month off Hawaii during the world's largest international maritime exercises even though environmentalists sued to stop the drills, citing potential harm to marine mammals.

The Defense Department on Friday granted the Navy a national security exemption that effectively enables the military to sidestep the lawsuit.

Environmental groups sued the federal government earlier this week, saying the sound from active sonar could harm whales and other marine mammals.

The Natural Defense Council filed a lawsuit to prevent the use of sonar, saying it would harm marine mammals that depend on echo-location to navigate.

The Navy has actually been given a six-month exemption to use sonar, which is long after Rim Pac exercises are over and would still be intact when humpback whales return to Hawaiian waters for the winter.

Navy officials said they will make sharp cuts in sonar power if a marine mammal is spotted near a ship. More than 40 ships from eight nations are already in Hawaii for the drills, but the sonar portion of the exercises are not due to begin until next week.