After several years of debate, the National Marine Fisheries Department on Monday granted the Navy permission to operate its new low-frequency sonar system, which is designed to blast wide areas of ocean with sound waves in an effort to detect submarines equipped with "stealth" technology.

The navy says the new sonar is needed to protect U.S. warships from a new breed of submarines that can barely be detected by conventional sonar systems.

The fisheries service, a division of the Commerce Department, said it was approving the sonar after determining that "marine mammals are unlikely to be injured by the sonar activities."

It further said that various "mitigation efforts", including provisions that the sonar cannot be used within 12 miles (19 km) of the coast and must shut down if any whales, sea turtles or other marine mammals are detected within about 1.2 miles (2 km), would also help to ensure that the sonar does not hurt marine mammal populations.

The sonar has been criticized by the NRDC and other environmental groups, which charge that it produces ambient noise levels that might physically harm whales and other marine mammals or alter migration or other behavior patterns seen as crucial to their survival.

"There's a growing body of evidence that indicates intense active sonar harms marine mammals, causing them to strand, causing physical injury, disruptions," Jasny said. "This could potentially affect the entire ocean environment."

Monday's decision by the fisheries service exempts the Navy from the Marine Mammal Protection Act, allowing it to operate the low-frequency sonar for a five year period, subject to annual review.

Under the exemption, the Navy will be allowed to "harass" small numbers of marine mammals by operating its sonar - permitting disturbances to animal behavior that would otherwise be barred by the 1972 act.