Conservation groups on Wednesday urged the central government and the U.S. military to assess the impact of the MV-22 Osprey crash off Okinawa Prefecture earlier this month in waters where the endangered dugongs live.

The Nature Conservation Society of Japan, Save the Dugong Campaign Center, and four other groups working for similar causes sent separate letters to the government and U.S. military in Japan to warn them that the crash "could have resulted in the plane hitting a dugong."

The U.S. Marine Corps MV-22, tilt-rotor aircraft based at Futenma air station in Ginowan, crashed on Dec. 13 in shallow waters off the prefecture's eastern coast.