TOKYO, Dec. 18 (Xinhua) -- An elementary school in Okinawa was offered an apology on Monday by U.S. military officials over a window falling from one of its helicopter last week and hitting the grounds where some children were taking a sports class.

According to local reports from Japan's southernmost prefecture, Col. Darin Clarke from the U.S. Marines' government and external affairs in the Pacific, apologized to Etsuko Kyan, principal of the Futenma Daini Elementary School.

Clarke said the incident had caused great unease in the school and the wider region, local reports said.

Although only one student sustained minor injuries as a result of the metal-framed window falling from the CH-53E transport helicopter, local concerns and anger were stoked by the accident, fueling rising anti-U.S. military sentiment on the island.

Okinawa Deputy Gov. Moritake Tomikawa told a press briefing Monday he had been informed that U.S. forces plan to resume flights of its CH-53E chopper, which had been grounded pending an investigation into the cause of the falling window.

The U.S. forces told both local government officials in Okinawa and the central government that the cause of the approximately 1-meter, 8-kg window falling from the chopper, was down to pilot error, with the verdict causing dismay and refutation among Japanese officials and citizens.

Okinawa Gov. Takeshi Onaga said the resumption of flights of the CH-53E helicopters was "truly ridiculous." He also accused the central government of being "incapable of being involved" as it failed to keep the flights grounded, while slamming the U.S. military as "not a good neighbor," according to local accounts.

Defense Minister Itsunori Onodera said the government has "not received a sufficient explanation about U.S. measures to prevent a repeat of the incident."

Parents of children attending the school were also quoted as saying they were disgusted at the flights being allowed to resume and that their restart was unforgivable and showed U.S. indifference to the people of Okinawa.

The U.S. side saying that pilot error was the cause of the window falling from the chopper, came despite local police officers in Okinawa being granted rare access on Thursday to the U.S. Marine Corps Futenma Air Station in Ginowan City, where the CH-53E transport helicopter is based, to conduct an investigation.

The inspection, overseen by the U.S. military, came as part of a probe to determine the cause of the window falling from the chopper and hitting the school's ground, narrowly missing around 50 children who were taking a physical education class on the playground at the time.

According to local media reports, the police said the window fell onto the grounds of the Futenma Daini Elementary School and landed around 10 meters away from where children from the 4th grade were taking a physical education class.

The school said that one child sustained slight injuries as a result of the falling window.

Okinawa Gov. Takeshi Onaga, at the time, described the incident as "unforgivable" stating that the safety of the school children should be a priority.

The elementary school is located on the north side of the controversial Marine Corps Futenma Air Station, which is located in the densely-populated city of Ginowan in Okinawa.

All CH-53E transport helicopters in Okinawa underwent inspections following the incident, an official from the U.S. Marines told public broadcaster NHK, and would be grounded until the chief of U.S. forces in Okinawa, Lieutenant General Lawrence Nicholson, confirmed their safety.

The Futenma base itself is central to an ongoing feud between the central government of Japan and local Okinawan government officials and citizens about its planned relocation to a coastal area within Okinawa.

Okinawa hosts the bulk of U.S. military bases in Japan, yet represents just a tiny percentage of Japan's land mass and anti-U.S. sentiment has been increasing on the small, subtropical island amid rising instances of U.S. base-linked crimes, noise, pollution and military accidents.

Calls from local officials for the Japan-U.S. status of forces agreement (SOFA) to be further changed to allow local authorities to seize and search U.S. military hardware and assets after accidents have become more vociferous since the latest helicopter-linked incident.

Investigations by Japanese authorities into the crash and potential leaking of low-level radiation, following the crash-landing of another U.S. military CH-53E helicopter near the U.S. military's Northern Training Area, just 300 meters away from a residential area in Okinawa, in October, were hampered by the current Japan-U.S. status of forces agreement, local officials said at the time.