The Moray Firth is well-known for its bottlenose dolphins
The moving of Europe's biggest military exercise to the outer Moray Firth poses a threat to bottlenose dolphins, conservations have claimed.
The Whale and Dolphin Conservation Society (WDCS) said noise from warships' sonar could harm the animals.
Exercise Joint Warrior involves Nato member countries and other allied armies, navies and air force.
The Royal Navy said it was obliged by law to use active sonar responsibly and not within the Moray Firth.
A spokesman said naval crews did not set out to harm marine life and sonar was used well below the maximum levels permitted during training.
He said crews complied with the law and followed guidance set out by the Joint Nature Conservation Centre, an advisor to the government on UK and international conservation.
Ships have been instructed not to use active sonar within 30 miles of the outer limits of the Moray Firth - a special area of conservation - and the Royal Navy said its calculations suggested the noise would be below stipulated levels by the time it reached the firth.
Held twice a year - in spring and autumn - the October training has been extended from the usual two weeks to three this year.
The exercises have been deemed necessary by the MoD and Nato for preparing personnel and equipment for combat missions.
The first week takes place in a tightly controlled area, but the remainder of the exercise sees the various forces spread out to prepare for a "full scale war" scenario.
In Scotland, training has usually taken place off the north west coast with Cape Wrath the scene of live firing by warships.
Last year, live firing was blamed for starting a heath fire at Cape Wrath.
WDCS said it was the first time the exercise had included the Moray Firth.
The area is well-known for bottlenose dolphins and people regularly gather at points along the coast of the firths furthest inland stretches to watch them.
Sarah Dolman, WDCS's noise campaign manager, said new generation sonar was powerful and a very loud noise source for marine life.
"We are extremely concerned that the dolphins in the Moray Firth will not be adequately protected from this by the mitigation measures proposed," she said.
"Exercise Joint Warrior is now expanding from the west coast and into the outer Moray Firth - into the region of the bottlenose dolphin's habitat and their European designated sanctuary, the special area of conservation."
Earlier this summer, hundreds of dolphins more commonly found in warmer seas were seen in the Moray Firth while making a "massive migration" into the North Sea.
The environmental charity Earthwatch Institute said more than 400 short-beaked common dolphins were sighted off the north east coast.
It said the "super pod" was a sign of how climate change was pushing some wildlife further north.
An Earthwatch team headed by Dr Kevin Robinson, director and co-founder of the Cetacean Research and Rescue Unit (CRRU), saw the pod 10 miles off land.
In a statement released before the exercise, the Ministry of Defence said potentially damaging activities, such as the use of active sonar and live weapons, were assessed during the planning stage.
Running until 22 October, 18 ships and three submarines had been expected to take part along with 60 aircraft.
Maritime patrol aircraft from Canada, France, Germany, Norway and the US had also been expected to participate - together with helicopters from Brazil, Canada, Denmark, Portugal, the UK and the US.
Next year's exercises are scheduled to take place on 12-23 April and 4-22 October.