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

The order comes 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 had sued to stop the Navy's use of the sonar in the Rim of the Pacific 2006 exercise off Hawaii. The use of sonar in the war games was set to start Thursday.

U.S. District Judge Florence-Marie Cooper wrote in her order that the plaintiffs "have shown a possibility that RIMPAC 2006 will kill, injure, and disturb many marine species, including marine mammals, in waters surrounding the Hawaiian Islands."

The exemption would have temporarily relieved the Navy from the requirements of the Marine Mammal Protection Act.

The Natural Resources Defense Council, which filed the lawsuit, said the Navy had more than enough room in the oceans to train without injuring marine life.

The National Oceanic and Atmospheric Administration, which forged an agreement with the Navy last week permitting the use of the sonar, concluded that the exercises would have no significant impact on the environment, under the National Environmental Policy Act.

NOAA's permit to use the sonar was the first time such a permit had been granted to the Navy.

NOAA determined that the exercise would cause no significant environmental impact, and concluded that the Navy's use of the sonar was not likely to jeopardize the continued existence of threatened and endangered species — including the Hawaiian monk seal — in the exercise areas.

But Cooper found, among other reasons, that the Navy violated NEPA requirements by not giving "full and meaningful consideration" to reasonable alternatives, including holding the exercises in less densely populated marine habitats.

Cooper's order was to remain in effect until July 18, when a hearing will be held on whether to replace the temporary restraining order with a preliminary injunction.