By Janet Williams BBC reporter in Seattle

A coalition of environmental groups led by the Natural Resources Defence Council is suing the US Navy and the National Marine Fisheries Service to prevent the deployment of a new, powerful sonar system.

The group filed a lawsuit in San Francisco, saying the system known as the Surveillance Towed Array Sensor System Low Frequency Active Sonar (Surtass-LFA) threatens entire populations of marine mammals, such as whales, dolphins and seals.

Environmentalists said a Navy sonar led to the Bahamas beachings in 2000

The US Navy says the new sonar, developed at a cost of $300m, is essential to its anti-submarine warfare effort and that its extensive studies show that it has a negligible effect on wildlife.

The Navy's new sonar emits powerful low-frequency sound to detect submarines at great distances, as far away as 450 kilometres (280 miles).

The effect, say environmentalists, is to saturate huge areas of ocean with extremely loud and disruptive sound. This noise, they say, harms and even kills marine mammals such as whales, which use their own sonar systems for feeding, communication and navigation.

Navy gets go-ahead

The environmentalists say that powerful mid-range sonar, used by the Navy off the Bahamas two years ago, caused 16 whales to run aground. Eight of them died.

The Navy disputes those findings. It says that its own extensive research into low-frequency sonar shows that few, if any, marine mammals would be subject to harmful noise.

The new system, it adds, is the only technology capable of detecting today's quieter diesel-powered submarines and is therefore vital for national security.

Last month the US National Marine Fisheries Service, the government agency responsible for protecting ocean wildlife, decided to allow the Navy to deploy the radar for five years. It was, said one environmental campaigner, a licence to kill.