WASHINGTON - THE United States' highest court has ruled that the US Navy can continue to use long-range sonar in exercises off the California coast, even if the practice confuses and deafens whales - or worse.

"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 Supreme Court said in an opinion written by Chief Justice John Roberts.

At the start of the hearing last month, government lawyer Gregory Carre acknowledged that a preliminary navy study found that sonar use in 14 naval exercises could disorient 170,000 marine mammals, and leave 8,000 whales temporarily deaf.

"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."

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

Environmentalists say such sonar use has potentially catastrophic consequences for marine life, arguing it confuses the animals and has caused mass deaths in the Bahamas and Canary Islands.

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

A few days later, the government granted the navy an exemption, saying the use of sonar was vital for military exercises that were in the "paramount interest" of the US.

However, environmentalists rejoiced that the Supreme Court did not rule on other measures such as the need to respect a protective zone off the California coast. -- AFP