WASHINGTON -- Federal regulators granted the Navy a permit on Tuesday to use sonar in a maritime exercise off Hawaii.

The approval came despite environmentalists' concerns the use of sonar could disturb or even kill whales and dolphins.

It was the first such permit granted to the Navy, and one environmental group, the Natural Resources Defense Council, said it would file a lawsuit on Wednesday to prevent the sonar's use.

The monthlong exercise, which includes anti-submarine training, involves naval forces from eight nations. It began Monday off the Hawaiian Islands. The sonar part of the exercise begins after July Fourth and lasts three weeks.

The National Oceanic and Atmospheric Administration gave the Navy a permit to use mid-frequency active sonar, which can affect marine mammals' behavior. In documents released on Tuesday, NOAA determined that the exercise would cause no significant environmental impact.