"There is now compelling evidence implicating military sonar as a direct impact on beaked whales in particular," said the report, released at the IWC's four-day annual convention which was winding up yesterday.

The report cited examples of bizarre and self-destructive whale behaviour that seemed to have been caused by military sonar, such as a mass stampede of 200 melon-headed whales into shallow water in Hawaii last month during a U.S.-Japanese naval training exercise. One animal died.

Scientists are unsure exactly why sonar causes whales to get stranded. One theory is that the noise disrupts their communication and navigation systems. Another is that the signals confuse whales in deep water, forcing them to surface quickly, suffering rapid decompression and a form of the bends.

The report may strengthen the hand of U.S. conservation groups which are threatening to sue the navy over its use of mid-frequency sonar.

BACK-FLIPS

The Natural Resources Defense Council (NRDC) has already secured an injunction limiting the U.S. Navy's use of new low-frequency sonar that can travel vast distances through the oceans, and is now targeting the more common mid-frequency sonar.

"This is the first time such a broad, diverse group (of scientists) has made this finding," said NDRC lawyer Joel Reynolds. "Navies of the world do back-flips to deny any connection."

The IWC, a 57-country intergovernmental body which regulates whaling, said earlier in the week that oil and gas exploration off Russia's Pacific coast threatened a colony of grey whales with extinction due to sonar and the threat of pollution.

Energy firms blast noise waves down to the sea floor to detect the presence of oil and gas reserves.

The IWC expressed concern about energy activities in the Sakhalin Island region off Russia where Royal Dutch/Shell, Exxon Mobil and BP operate.

Its scientific report also identified oil and gas exploration near the Abrolhos Banks, a coral reef off Brazil, as a hazard for humpback whales and called on the government to protect the mammals from the noise.