The US Navy is refusing to detail its sonar use for a US court in a case involving potential harm to whales, saying the information could jeopardise national security.

Navy Secretary Donald Winter said the information requested by the plaintiffs - the Natural Resources Defense Council (NRDC) was classified and its disclosure "could reasonably be expected to cause exceptionally grave damage to national security." Active sonar, which the world’s navies use by pumping sound through water and listening for objects the sound bounces off, has been known to kill marine mammals. A U.S. Congressional Research Service report last year found Navy sonar exercises had been responsible for at least six mass deaths and unusual behavior among whales. Many of the beached or dead animals had damaged hearing organs.

U.S. District Judge Florence-Marie Cooper issued an order for the Navy to submit data for the case on when and where their sailors have used sonar since 2003. Cooper instructed the Navy to list the latitude and longitude of where it used mid-frequency active sonar, how long it used the sonar, and the times and dates. However they are refusing to comply citing state secrets privilege, which allows government officials to keep information secret on national security grounds. The state secrets privilege is well-established US federal law and has been recognised by U.S. courts as far back as the 19th century,

Joel Reynolds, a NRDC lawyer said they would challenge the Navy's position as it’s vital to have the data to ascertain how the Navy has complied with environmental laws when sailors use sonar. In January, the Pentagon exempted the Navy for two years from a law protecting whales so that it could continue using the sonar during training. That removed one legal avenue for environmental groups to challenge Navy sonar.

The sonar systems employed by the Navy are increasingly implicated as a source of major damage to marine mammals from whales to dolphins.