Accidents or incidents involving U.S. military aircraft in Japan more than doubled to 25 cases in 2017 from 11 the year before, Japanese Defense Minister Itsunori Onodera said Monday.

Onodera said the Defense Ministry conducted an assessment of the issue after he was told by U.S. Pacific Command chief Adm. Harry Harris earlier in the month in Hawaii that the number of accidents had decreased in 2017.

"Based on the information we have at hand, (accidents are) increasing," Onodera told reporters. Another senior official said the ministry is not aware of what data Harris was referring to during his talks with Onodera on Jan. 9.

Harris said at the outset of the meeting that was open to the press that the number of incidents had decreased from more than 30 in 2016 to around 25 in 2017, including minor cases, serving as proof of the priority the U.S. military has been putting on safety.

The safety of U.S. aircraft and other military operations is a constant source of tension in Okinawa, which bears the heaviest weight of the U.S. forces' presence in Japan.

Political pundits say that with a local election looming, Onodera's remarks may be a show of support from the Japanese government for the people of Okinawa, who have been angered by the frequency of accidents and incidents involving U.S. military aircraft.

The outcome of the Feb. 4 mayoral election in Nago, where U.S. Marine Corps Air Station Futenma is planned to be transferred from a crowded residential area of Ginowan, could affect the ongoing controversial base relocation plan.

The airfield relocation to the less populated Henoko coastal area of Nago is intended to put an end to safety issues caused by operations at the Futenma base. But Okinawa Gov. Takeshi Onaga and many local residents want the facility relocated outside the prefecture.

Most recently, the Japanese Defense Ministry and the U.S. military have been at odds over whether the military flew its helicopters above an elementary school near the Futenma base.

According to the ministry, local defense bureau officials stationed at the Futenma No. 2 Elementary School said they spotted three helicopters flying over the school Thursday, despite a U.S. promise to avoid such flights after a window fell from another chopper into the grounds of the school in December.

The ministry has also provided to the U.S. military security camera footage taken at the school to back up its argument. But the U.S. military insisted that no Marine Corps aircraft were confirmed to have flown over the school based on radar tracking data and pilot interviews.

The U.S. military has also said the helicopters were more than 100 meters away from the school during the controversial flight, a Japanese government official said Monday.

According to the radar data, shown to an Okinawa prefectural assembly member the same day, helicopters were flying in an area between the premises of the Futenma No. 2 Elementary School and Futenma Junior High School. But the two schools are only 120 meters from each other, the Ginowan city government said.

The Dec. 13 incident, which did not result in any injuries, is included in the 25 Japan tallied in its assessment. The number also includes a crash-landing of another CH-53E helicopter near the U.S. military's Northern Training Area on Okinawa's main island last October.

Earlier in January, there were two other forced landings by U.S. military helicopters in Okinawa.