An environment group is considering going to court to stop the US Navy from using a new sonar system that could harm whales and other marine mammals.

Last week, the US National Marine Fisheries Service granted the Navy permission to operate its new low-frequency sonar system, the Surveillance Towed Array Sensor System, said to be more sensitive and far-reaching than current systems.

It works by blasting wide areas of ocean with sound waves to try and detect a new type of submarine. The submarine is almost impossible to discern using conventional sonar systems, and the Navy argues that it needs the new system for national security.

But the Natural Resources Defense Council and other environment groups are concerned that the low frequency sonar will injure marine mammals, or affect migration or other behaviour patterns crucial to their survival such as sense of direction, communication and the ability to detect predators and prey.

"There's a growing body of evidence that indicates intense active sonar harms marine mammals," said Michael Jasny, senior policy analyst with the Natural Resources Defense Council.

In March 2000 seven whales died after a beaching on the Bahamas coast, close to the time and location of a US Navy sonar operation. The whales that died were found to have inner ear damage, which scientists said might have been caused by the Navy's powerful sonar technology.

The Fisheries Service said it had approved the new sonar after determining that marine mammals were unlikely to be injured by the sonar activities.

Various mitigation efforts are intended to protect mammal populations, including provisions that the sonar cannot be used within 19 kilometres of the coast and must be shut down if any whales, dolphins or other marine mammals are detected within 2 kilometres.

According to Colin Trinder, director of Environmental Stewardship for Australia's Commonwealth Department of Defence, the Royal Australian Navy "doesn't use these sonar systems and we wouldn't be inviting foreign nations to come here and use that technology in Australia".

He said that as a result of the Bahamas incident, Australia has re-examined its own mitigating procedures during sonar operations in Australian waters.

These include aerial surveillance to determine whether there are any marine animals within the vicinity of a planned operation, not using sonar in confined spaces, and 'ramping up' the sonar transmission.

"This means turning up the volume gradually which gives animals a chance to get away," he said.