The US Supreme Court has ruled the US Navy can continue to use long-range sonar in exercises off the California coast, dismissing arguments that the practice was harmful to whales.

"Even if the plaintiffs have shown irreparable injury from the navy's training exercises, any such injury is outweighed by the public interest and the navy's interest in effective, realistic training of its sailors," the court said in an opinion written by Chief Justice John Roberts.

In a split decision, five of the nine Supreme Court judges agreed with the Government's case that, in the interests of national security, President George Bush has the constitutional power to exempt the US Navy from environmental laws curbing the use of long-range sonar in the North Pacific.

"We do not discount the importance of the plaintiffs' ecological, scientific, and recreational interests in marine mammals," the opinion said.

"Those interests, however, are plainly outweighed by the navy's need to conduct realistic training exercises to ensure that it is able to neutralise the threat posed by enemy submarines."

A navy spokesman did not wish to comment on the decision.

The navy uses sonar off California to look for hostile submarines lurking beneath the Pacific, but has duelled with environmentalists for years in federal courts over its use.

Environmentalists say such sonars have potentially catastrophic consequences for marine life, arguing they disorientate the animals and have caused mass deaths in the Bahamas and Canary islands.

In January, a court required the navy to take safety precautions off the California coast inhabited by five species of endangered whales.

A few days later, Bush riposted by granting the navy an exemption, arguing the use of sonars was vital for military preparedness exercises that were in the "paramount interest of the United States."

Environmentalists took their case to the Ninth Circuit Court of Appeals, which upheld the lower court's decision in February.

But the government then petitioned the Supreme Court.

And in today's ruling the Supreme Court found the lower court had exceeded its authority in telling the navy to reduce sonar levels.

At the start of the Supreme Court hearing in October, government lawyer Gregory Carre acknowledged that a preliminary navy study found that sonar could disorient 170,000 marine mammals, and leave 8000 whales temporarily deaf.

But he defended the sonar level used by the navy as being well below the danger level for marine life.

The environmentalists' argument was put by the National Resources Defence Council (NRDC), a Washington-based environmental action group that claims 1.2 million members.

"This case is about achieving environmental protection while maintaining our important national security standards," Jeffrey Flocken, Washington director of the International Fund for Animal Welfare, said in an October NRDC statement.

"It does not need to be an either-or scenario when it comes to ensuring our waters are protected and our marine wildlife is healthy."

Two judges on Wednesday insisted however that while there was "no doubt that the training exercises serve critical interests ... those interests do not authorise the navy to violate a statutory command."

Dissenting judge Ruth Ginsburg further said the navy had failed to obey the law by not supplying a full report on the environmental risks before beginning to use sonar, and just sticking to its preliminary report.

AFP