The United States National Resource Defence Council says the US Navy's mitigation plans are inadequate to prevent harm to marine mammals from high-energy sonar training in the Northern Marianas.
The Navy says it plans lookout surveys and safety zones around training vessels and monitor these zones for marine animals.
It also says that ships will power down sonar when an animal is spotted within a kilometre of the vessel, with an additional power down at 500 metres, and a full shut down at 200 metres.
However, the NRDC says studies show that the Navy only has a 1-in-50 chance of spotting marine mammals in the zone, as they are cryptic species that produce little spray when surfacing water.
The sea around the Northern Marianas is believed to hold naive and relatively under-researched stocks of marine mammals.