WASHINGTON — The Defense Department granted the Navy a national security exemption Friday for the use of sonar during a maritime exercise, sidestepping a lawsuit that sought to protect whales from the noise.

Environmental groups sued the federal government on Wednesday to prevent the Navy from using sonar during maritime exercises off Hawaii, saying the sound could harm whales and other marine mammals.

The exercise began this week, but the sonar portion of it will not begin until after Tuesday, the Navy has said.

The six-month exemption will allow the exercise to continue without a permit from U.S. regulators. And it will exempt the Navy from any requirements of the Marine Mammal Protection Act for that time.

Rear Adm. James Symonds, director of environmental readiness, said the Navy will continue to abide by the permit agreement forged with the National Oceanic and Atmospheric Administration earlier this week.

"The Navy will continue to employ stringent mitigation measures to protect marine mammals during all sonar activities, to include habitat controls, safety zones around ships, trained lookouts, extra precautions during chokepoint exercises, in coordination with National Marine Fisheries Service," he said.

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

"This is an historic and unprecedented retreat by the U.S. Navy from our national commitment to protect whales, dolphins and other marine life," said Joel Reynolds, a senior NRDC attorney. "It's not that the Navy can't comply with the law; it's that the Navy chooses not to."

Symonds said the exemption period will give the Navy time to work with NOAA on long-term plans to comply with marine mammal protections and other environmental requirements. He said the exemption affects 12 more exercises — which are each two to three weeks long — over the next six months on both coasts. Each would involved about a dozen ships.

NOAA on Tuesday granted the Navy a permit to use mid-frequency active sonar, which can affect marine mammals' behavior. It was the first time such a permit had been granted to the Navy. The monthlong exercise, which includes anti-submarine training, involves naval forces from eight nations.

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.

As part of the permit agreement, the Navy said sonar operators will reduce active sonar power by 75 percent if a marine mammal is spotted within 1,094 yards of the ship, and drop it further if mammals are detected within 547 yards. They will turn off sonar if the mammal is detected within 219 yards.

The exemption came after the NRDC asked a federal court in Los Angeles to issue a temporary restraining order unless the Navy takes "effective measures" to comply with legal protections for marine life when it uses high-intensity, mid-frequency active sonar to hunt submarines in the drills.

Under the defense authorization law for fiscal year 2004, the secretary of defense, after consultation with the secretary of commerce, has the authority to approve such exemptions for national security reasons.