The NRDC and four other environmental groups sued to block the deployment, arguing that the loud, low-frequency sonar for detecting submarines at great distances would harm and kill whales, dolphins, seals and other marine mammals.

Under an agreement reached on Friday between the Navy and the environmental groups, the Navy will be able to test the sonar for seven months in only one million square miles (2,6 million square km) in a northwestern section of the Pacific, Reynolds said.

The remote testing area is west of Hawaii, east of Japan and Taiwan and north of the Philippines, he said.

"This is an area that is not likely to have a high presence of marine mammals," Reynolds said. "In particular, it's away from feeding and breeding areas and whale migration routes."

Many marine mammals need their hearing to find food and mates and to avoid predators. Navy studies have found that the new sonar system generates sounds capable of reaching 140 decibels as far away as 300 miles (480 km), the environmental groups say.

Navy public affairs officers did not return calls for comment on Saturday.

Late last month, a judge in San Francisco granted a preliminary injunction barring the Navy from using the sonar, ruling that the permit likely violated several U.S. environmental protection laws, Reynolds said.

The judge agreed to allow temporary testing of the sonar in a limited area pending the final decision after the Navy cited national security concerns, he said.

Reynolds said he expected the trial to begin in June and a final decision in July.

A federal investigation identified Navy sonar as the cause of a mass stranding of whales in the Bahamas in 2000 and the disappearance of an entire population of beaked whales in the region, according to environmental groups.

In the beached whales that died, scientists found hemorrhaging near the brain and ear areas, which are injuries consistent with exposure to high-intensity loud noise, environmentalists said.

In a separate case, a San Francisco federal judge last month ordered the National Science Foundation to stop firing loud sonic blasts from air guns in the Gulf of California. The blasts, part of seismic research in Mexico, have been linked to the death of whales, according to environmental groups.