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Part of a US helicopter has crashed on a school in Okinawa, Japan, renewing tensions with the local population.

The window dropped on the school grounds, slightly injuring one boy, news agency Kyodo said.

The southern island of Okinawa hosts the largest US military presence in Japan.

Over the past years, a number of accidents and crimes have led to growing local opposition to the US base.

According to the Reuters news agency, 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".

The US military that the window of one of its helicopters fell onto the sports ground of an elementary school outside the US Marine Corps Air Station Futenma.

"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."

The US presence on Okinawa is a key part of the post-war security alliance between the two countries.

The Futenma base is located in a residential area, with schools, shops and hospitals. Residents have long been concerned over possible crashes or accidents.

In October, another US helicopter crash landed and burst into flames in northern Okinawa.

In November, a US serviceman caused a deadly car crash on the island. When the accident was linked to drink driving, the US military banned all soldiers stationed in Japan from drinking alcohol.

In 2016, the murder of a woman was linked to an ex-Marine employed at one of the bases, also leading to a temporary ban on alcohol as well as a midnight curfew.

The US government wants to move Futenma, one of several Marine bases in Okinawa, to a more remote location on the island.

But the governor of Okinawa, Takeshi Onaga is leading a campaign to get the US military base off the island completely.