TOKYO -- Japanese officials said a boy was injured in Okinawa on Wednesday when a metal window frame fell from a U.S. military helicopter, the second such incident in the past week on the southern island.

Officials from the city of Ginowan said the window fell from a CH-53 transport helicopter and landed on a school playground, leaving a boy with minor arm injuries. About 50 children were outside the school next to the Marine Corps Air Station Futenma, at the time.

Last week, a part of another U.S. military helicopter fell on a nearby kindergarten roof, but no one was injured.

"It's outrageous this happened again," Okinawa Gov. Takeshi Onaga told reporters angrily. "Despite repeated accidents, U.S. military officials would not lend their ears to our concerns."

Get Breaking News Delivered to Your Inbox

Brig. Gen. Paul Rock, who oversees Marine Corps installations in the Pacific region, said later Wednesday that all CH-53 helicopters at the Futenma base have been grounded for safety checks when he visited the prefectural government to offer an apology over the incident.

The base in a crowded residential area in central Okinawa is a source of anti-U.S. military sentiment and safety concerns. Its planned relocation pushed by the Japanese and U.S. governments has been delayed for more than 20 years as many residents want it entirely off of Okinawa.

The latest incidents could further fuel opposition to the U.S. military presence on Okinawa, where about half of 50,000 American troops are stationed and where local residents have expressed concerns about military crime and safety on the island.

Chief Cabinet Secretary Yoshihide Suga urged the U.S. military to be more attentive to safety and make more efforts to minimize its impact on the local community.

A Marine Corps CH-53 made an emergency landing on a farm and burned in October. Another helicopter belonging to Futenma crashed into a nearby university in 2004, injuring three U.S. crew members.