The Defense Department gave the Navy permission Tuesday to keep training with sonar for another two years, a move denounced by activists who say the sound waves can harm dolphins and other marine mammals. Navy officials had sought the two-year exemption from the Marine Mammal Protection Act, allowed under the 2004 National Defense Authorization Act. The ranges are off Hawaii, Southern California and the East Coast. We cannot stop training for the next two years," said Don Schregardus, deputy assistant secretary of the Navy for the environment. "That would put our sailors in the Navy at considerable risk." Environmentalists cite incidents of whales, porpoises and dolphins that have become stranded en masse on beaches after being exposed to sonar. "The Navy has more than enough room in the oceans to train effectively without injuring or killing endangered whales and other marine species," said Joel Reynolds, senior attorney at the Natural Resources Defense Council, which is suing the Navy over its sonar use. Navy officials said they're claiming a two-year exemption because a federal judge in California ruled last year that the Navy needed to do more detailed analysis of the effect its sonar training would have on the environment. The Navy says the new exemption will allow sailors to go ahead with 40 separate exercises over the next two years.