Mishaps involving U.S. military aircraft in Japan more than doubled in 2017 from 2016, Defense Minister Itsunori Onodera said Monday.

Onodera said the Defense Ministry counted 25 cases last year, compared with 11 in 2016.

"Based on the information we have at hand, they're increasing," he told reporters.

After two emergency landings involving U.S. helicopters in Okinawa Prefecture, Onodera met U.S. Pacific Command chief Adm. Harry Harris in Hawaii on Jan. 9 and urged him to work to prevent similar incidents.

Harris said at the outset of that meeting, which was open to the media, that the number of accidents had declined, serving as proof of the priority the U.S. military has placed on improving safety.

The safety of U.S. aircraft and other military operations is a constant source of tension in Okinawa, which is home to the majority of U.S. forces in Japan.

In December, a window fell from a U.S. chopper onto the playground of an Okinawan elementary school near unpopular U.S. Marine Corps Air Station Futenma, which is situated in a crowded residential area of Ginowan, nearly hitting a group of pupils.