TOKYO (Reuters) - Japan criticized the U.S. military on Friday for breaking a pledge to avoid flying helicopters over a school next to its base in Okinawa, a rare flash of discord with Tokyo’s main ally.

The school next to the Futenma base on the southern island of Okinawa has become a focal point for discontent over the U.S. presence on the island since a window fell from a U.S. Marines helicopter onto its playground last month.

The Marines blamed the incident on ground personnel failing to secure the window properly and promised to try to fly around the school in future.

A string of recent aircraft incidents, including the falling window and emergency landings by two helicopters this month, have spurred renewed calls in Okinawa for the Americans to close some or all of their bases.

“It is unacceptable. We would like the U.S. to deal with this,” Japan’s Minister of Defence Itsunori Onodera said.

His ministry had clear evidence of Thursday’s overflight, including radar data and video, which it had given to the Marines and released to the media, Onodera said.

The Marines insisted that their helicopters had avoided the school.

“Pilots from yesterday’s flights were aware of the school’s location and avoided it, and Marine Corps Air Station Futenma confirmed using radar tracking data and pilot interviews that no Marine Corps aircraft flew over the school yesterday,” they said in a statement.

Teachers and students at the elementary school conducted drills yesterday to evacuate the playground in case of falling aircraft parts.

Located strategically at the edge of the East China Sea, Okinawa hosts some 30,000 military personnel living and working on bases that cover a fifth of the island. Okinawa was under U.S. occupation until 1972.