A window fell from a U.S. military transport helicopter onto the grounds of an elementary school in Okinawa on Wednesday, triggering renewed concern in the prefecture that hosts the bulk of U.S. forces in Japan.

The 1-square-meter metal-framed window, weighing about 7.7 kilograms, dropped from the CH-53E chopper while some 60 students were taking physical education classes in the playground of a school in the city of Ginowan, located just outside U.S. Marine Corps Air Station Futenma, local officials said.

According to the school, the students ran to a nearby building after the incident, some of them crying. A boy in the fourth grade was hit in the arm with gravel thrown up as the window frame struck the ground, the local police and other sources said. There were no obvious marks on him, however.

It is "a miracle" that the incident did not cause any serious injuries to students, said Etsuko Kyan, the principal of the Futenma Daini Elementary School. She said the children are so upset that the school cannot resume physical education classes anytime soon.

The U.S. Marine Corps apologized for the incident and Brig. Gen. Paul Rock, commanding general of Marine Corps installations in the Pacific, told the Okinawa prefectural government that all the CH-53Es that belong to the Futenma base have been grounded for safety checks.

Okinawa Gov. Takeshi Onaga reacted sharply to the incident, the latest in a series of mishaps involving U.S. military aircraft based in Futenma.

"The safety of children should come first. It is unforgivable that it dropped in the middle of the playground," he told reporters as he visited the site. In Tokyo, Chief Cabinet Secretary Yoshihide Suga said the incident "stirred concern" among the people of the island prefecture.

The window, apparently acrylic, along with its metal frame fell onto the playground of the elementary school around 10:10 a.m. The nearest student was only about 5 meters away, according to a group of city assembly members who went to the site.

Shattered pieces of the window were scattered at the site, while parents, worried about their children, rushed to the school. The students in the playground at the time were in the second and fourth grades.

The Defense Ministry's local bureau told Okinawa Deputy Gov. Moritake Tomikawa that the window had fallen from the left side of the helicopter's cockpit. The CH-53E is a large transport helicopter, which can carry up to 55 personnel in addition to seven crew members.

Tomikawa also met Rock and called for emergency checks of all the U.S. military aircraft deployed in the prefecture and the suspension of their flights until the checks are complete.

The incident occurred less than a week after a small cylindrical object was found on the roof of a nursery school in Ginowan, with local government and the school officials believing it fell from a U.S. military aircraft.

The U.S. Marine Corps has admitted the object was part of a CH-53 helicopter but denied that it fell off during flight.

Given the conflicting opinions, the nursery school has received dozens of telephone calls and emails criticizing it for "telling a lie" or "trying to make up a story."

Takehiro Kamiya, the nursery school's principal, said Tuesday, "I don't want the prefectural and central governments to accept without question what the U.S. military says. I want people on the mainland to think of this as their problem."

The Japanese and U.S. governments struck an accord in 1996 on the return of land used for the Futenma base after public anger over the 1995 rape of a local girl by three American servicemen. But progress has been slow, with many locals opposed to the current plan to relocate the base within Okinawa.

Under the plan, the Futenma base is expected to be moved from a densely populated area of Ginowan to the less populated Henoko coastal district of Nago.

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

On the same day a year ago, one Osprey crash-landed off the coast of Nago in Okinawa, with two of the five crew members injured.

In 2004, a CH-53D helicopter from the Futenma base crashed at a university in Ginowan, injuring three crew members.