A federal judge in California has ordered the US Navy to temporarily stop using sonar equipment because it might harm whales and other sea mammals.
Whales use sound waves to navigate, hunt and communicate
Environmentalists applied for the restraining order to cover a Pacific warfare exercise off Hawaii's coast.
The US Department of Defense had earlier exempted the navy from another law aimed at protecting sea mammals against the use of sonar equipment.
Government lawyers were reviewing the ruling, a naval spokesman said.
Some scientists believe the powerful sound waves emitted by underwater sonar equipment can harm sea mammals.
The navy is carrying out the anti-submarine warfare training exercise, known as Rim of the Pacific (Rimpac) 2006, this week.
It involves 40 ships and six submarines, and the navy was planning to use a high-powered military sonar.
On Friday, the US Department of Defense for the first time gave the navy a six-month exemption from the Marine Mammal Protection Act, to allow the use of its sonar equipment.
But California district judge Florence-Marie Cooper based her order on the National Environmental Policy Act, after campaign group the Natural Resources Defense Council challenged the military exercise.
She wrote that the plaintiffs "have shown a possibility that Rimpac 2006 will kill, injure, and disturb many marine species, including marine mammals, in waters surrounding the Hawaiian Islands".
She said the navy should have considered holding the exercise in a less densely populated marine habitat.
"Fortunately this country has more than one law against the needless infliction of harm to endangered whales and the environment," Joel Reynolds, lawyer for the National Resources Defense Council, was quoted as saying by Reuters news agency.
Judge Cooper's order is due to remain in effect until 18 July when the Navy will be allowed to argue against the injunction at a full court hearing.
Rimpac 2006 involves eight countries: Australia, Britain, Canada, Chile, Peru, Japan, South Korea and the United States.