The Aegean Sea has seen a "very unusual" spike in dolphin deaths over the past few weeks, a Greek marine conservation group said Monday, adding that the rise could be linked to massive Turkish naval exercises in the area.

Fifteen dead dolphins have washed up on the eastern island of Samos and other parts of Greece's Aegean coastline since late February, according to the Archipelagos Institute.

Its head of research, Anastassia Miliou, told The Associated Press that 15 is a worryingly high number compared to "one or two" in the same period last year.

The group said while it's still unclear what caused the deaths, the spike follows the Feb. 27-March 8 Turkish "Blue Homeland" exercises — the country's largest ever — that made constant use of sonar and practiced with live ammunition.

The deafening noise of sonar, used by warships to detect enemy submarines, can injure dolphins and whales, driving them to surface too fast or beach themselves — with sometimes fatal consequences — to escape the din.

"We can't say that the Turkish exercises killed the dolphins, but the fact that we had such an unusual increase in the number washed up dead — and what we have seen must be a small percentage of the total because the Aegean has a long coastline — coincided with exercises that used more than 100 ships," Miliou said. "(The deaths) are very worrying and we can't say that the two events are unrelated."

After several mass beachings of whales, NATO, to which both Turkey and Greece belong, has adopted a code of conduct for using sonar that is designed to better protect marine mammals.

Miliou said the alliance's rules are respected by the U.S. Navy, which has a strong Mediterranean presence, and the Greek navy.

"There's a new (military) generation in place that doesn't want to harm the environment in the name of national sovereignty," she said.

But Miliou said it was unclear whether Turkey has implemented the guidelines. She said the Greek government in Athens should raise the issue in talks with Ankara, particularly ahead of new Turkish naval exercises in the Aegean.

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"With these giant exercises... strain is placed on the entire marine ecosystem, including fish and plankton, because (they) cause intense noise pollution from which marine life can't escape," she said.

Miliou added that the Aegean Sea "can barely handle the strain we are already subjecting it to," such as pollution, overfishing and heavy marine traffic.