A window from a US military helicopter fell onto a school sports ground in southern Japan Wednesday, the marines said, apologising for a "regrettable" incident likely to fuel opposition to their presence.

There were no reports of serious injuries from the accident that took place at 10.09am (0109 GMT, 9.09am Malaysia) local time at an elementary school near the Futenma marine airbase.

The US military said it was taking the incident "extremely seriously" and was opening an investigation.

"This is a regrettable incident and we apologise for any anxiety it has caused the community," the military added in a statement, urging local residents to stay clear from where the object landed "for safety purposes".

The incident comes just two months after an American military chopper burst into flames after landing in an empty field in Okinawa.

chopper (=helicopter)：ヘリコプター
チョッパー (Chopper)　- 英単語 "Chop" の "-er" 形。 "Chop (チョップ)" は、「叩き切る」「切り刻む」「短縮する」などの意味である。転じて「切り刻むもの」