TOKYO (Reuters) - A window fell from a U.S. helicopter onto a school sports field near a U.S. Marines air base on Okinawa on Wednesday, the Marines said, the latest in a series of accidents that have fanned safety concerns on Japan s southern island. Japan s central government and Okinawa authorities have been at odds for years over the Futenma base. Residents complain about what they see as the unfair burden they carry in supporting the U.S. military presence in Japan. A 10-year-old boy suffered a minor injury but the exact cause was unclear, an Okinawa prefecture official told Reuters. The boy was among about 50 children on the elementary school grounds when the window fell from a U.S. CH-53E transport helicopter. Japan s Chief Cabinet Secretary Yoshihide Suga said:  This sort of incident creates anxiety not only among those involved with the school but the people of Okinawa and should never happen.  Suga told a regular news conference that the government would respond appropriately after getting an explanation from U.S. officials. The U.S. Marines on Okinawa said in a statement the helicopter had immediately returned and reported the incident.   We take this report extremely seriously and are investigating the cause of this incident in close coordination with local authorities,  the statement said.  This is a regrettable incident and we apologize for any anxiety it has caused the community,  it said.   The Futenma base is surrounded by schools, hospitals and shops, and residents worry about air crashes. Crime committed by U.S. servicemen has also occasionally angered residents. Okinawa Governor Takeshi Onaga has led the campaign to get the base off the island, while the central government has proposed moving it to a less populated part of the island called Henoko. A U.S. Marines CH-53E helicopter made an emergency landing and burst into flames in a U.S. military training area in northern Okinawa in October. No one was injured. 