Okinawa's governor protested to the defense and foreign ministries on Thursday after a metal-framed window fell from a U.S. military helicopter onto the playground of an elementary school the previous day.

"It is absolutely intolerable," Okinawa Gov. Takeshi Onaga told Senior Vice Defense Minister Tomohiro Yamamoto in Tokyo. He also demanded all U.S. military aircraft in the southwestern prefecture be grounded until inspections are completed.

Onaga made similar remarks in a meeting later Thursday with Senior Vice Foreign Minister Masahisa Sato. Chief Cabinet Secretary Yoshihide Suga said he will meet with the governor on Friday.

Local anger was reignited by news that a roughly 90-square-centimeter window weighing 7.7 kilograms fell Wednesday from a CH-53E transport helicopter onto the playground of an elementary school adjacent to the U.S. Marine Corps Air Station Futenma. Around 60 students were participating in physical education activities on the playground at the time.

The window landed only a dozen meters away from the students and could have caused severe injury if it had hit anyone, the local police and other sources said.

"It is an accident unheard of. I want the U.S. side to take the accident seriously," Katsutoshi Kawano, chief of the Japanese Self-Defense Forces Joint Staff, told a press conference.

Kawano said he requested Lt. Gen. Jerry Martinez, commander of U.S. Forces Japan, by phone to make sure such an accident never happens again.

The Marine Corps has grounded all CH-53Es based at Futenma for safety checks but a number of other helicopter types were seen departing the base in the morning.

Okinawa prefectural police were allowed on Thursday to enter the base, controlled by the United States under the Japan-U.S. Status of Forces Agreement, and to see the status of the chopper, the police said.

But it remains unknown to what extent the United States will cooperate in the local police investigation into the accident.

Etsuko Kyan, the principle of the Futenma Daini Elementary School, the site of the accident, has decided to bar students from using the playground, according to the local municipal education board. She is urging the Japanese government to consider ways to prevent U.S. military aircraft from flying over the playground.

A second-grade student at the elementary school took leave Thursday, saying "(the incident) was scary" and complaining of ill health, the local municipal education board said.

The accident came less than a week after a small cylindrical object was found on the roof of a nursery school in Ginowan, which local government and school officials believe also fell from a U.S. military aircraft.

Local residents are frustrated by noise, crime and accidents linked to U.S. bases. Safety concerns were recently reignited by a series of accidents involving U.S. Marines Osprey tilt-rotor aircraft and another CH-53E helicopter belonging to the Futenma base.