Washington  - The US Supreme Court ruled Wednesday to eliminate restrictions on the Navy's sonar use in training exercises off the California coast.

The 5-4 decision by the high court was a blow to environmental groups who argued the practice harms sea life and causes some species of whales to beach themselves.

The Supreme Court determined the restrictions hamper the Navy's ability to detect increasingly sophisticated enemy submarines that could threaten the fleet, but offered no opinion on the damage the sonar could inflict on marine mammal life.

"We do not discount the importance of plaintiffs ecological, scientific, and recreational interests in marine mammals," Chief Justice John Roberts said in writing the majority's opinion.

"Those interests, however, are plainly outweighed by the Navys need to conduct realistic training exercises to ensure that it is able to neutralize the threat posed by enemy submarines," Roberts said.

The Navy and environmental groups have been sparring for years over the health effects of the sonar on sea life. The Navy says it already takes steps to minimize the impact on whales, dolphins and other mammals and that the training is vital to national security.

The National Resources Defence Council had brought the lawsuit and successfully persuaded lower federal courts in California to impose restrictions.

A federal court in San Francisco ruled in February that the Navy must adhere to rules on the powerful sonar for tracking submarines, including establishing a buffer zone within 20 kilometres of the California shore, and shutting the system down if marine mammals are spotted within 2,000 metres of the ships.

The Supreme Court concluded the lower court "abused its discretion" by imposing the restrictions.

The Navy says it has instituted 29 measures to protect sea life, including lookout stations, passive acoustic monitoring for marine mammals and the setting up of safety zones to reduce the sonar power or shut it down if marine life is spotted. (dpa)