HONOLULU -- A Los Angeles federal judge issued a temporary restraining order on Monday, barring the Navy from using a particular kind of sonar allegedly harmful to marine mammals during a Pacific warfare exercise scheduled to begin later this week.

Judge Florence-Marie Cooper made the order three days after the Navy obtained a six-month national defense exemption from the Defense Department allowing it to use mid-frequency active sonar.

Environmental groups sued to stop the Navy's use of the sonar in the Rim of the Pacific 2006 exercise off Hawaii.

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.

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 was not due to begin until Thursday.