A lawsuit aimed at protecting marine mammals is forcing a change of plans by the US Navy.

The Navy says it will rely on passive sonar during the Rim of the Pacific exercises starting tomorrow.

A federal judge in Hawaii has stepped in with a temporary restraining order, blocking the use of a high-intensity sonar known as "mid-frequency active sonar."

Environmentalists had accused the Pentagon of doing an end run around their lawsuit by granting the Navy a six-month exemption from the federal Marine Mammal Protection Act.

The commander of the Third Fleet, Vice Admiral Barry Costello says keeping ships, helicopters, airplanes and submarines from using the high-intensity sonar compromises the exercise's realism.

Environmentalists allege the sonar has the potential to kill, injure and disturb whales and other marine mammals.