Protesters staged a rally on Sunday on the Japanese island of Okinawa because of safety concerns about military aircraft at an American airbase.
They were demanding the scrapping of plans to station 12 MV-22 Osprey transport planes at a marine base in the middle of a heavily populated area.
The aircraft - which takes off like a helicopter - has been involved in two fatal crashes this year, in Morocco and Florida.
An MV-22 made an emergency landing in a residential area outside a Marine base in Jacksonville, North Carolina, on Thursday.
Japanese media report that Prime Minister Yoshihiko Noda told US Secretary of State Hillary Clinton of Japan's concerns over the Osprey's safety on the sidelines of an Asia-Pacific summit in Russia on Saturday.