A window fell off a U.S. military helicopter Friday at a U.S. base southwest of Tokyo, but no one was injured, Japanese Defense Minister Itsunori Onodera said.

The incident at the U.S. Atsugi base in Kanagawa Prefecture follows a similar incident last December, in which a window of a U.S. military helicopter fell onto an elementary school playground in the southern island prefecture of Okinawa.

"While no one was injured (on Friday) and it's a case that happened inside a U.S. military base, a window falling is a serious problem," Onodera told reporters.

In the latest case, the window fell from a U.S. Navy MH-53E inside the premises of the U.S. Naval Air Facility Atsugi at around 11:50 a.m. after taking off for refueling, according to the U.S. military and officials of a municipality hosting the base.

It dropped from around a height of 2 to 3 meters, they said. The helicopter will stay at the base for checks for the meantime, they said.

The Japanese Maritime Self-Defense Force, which jointly uses the Atsugi base with the U.S. military, noticed that a part fell from a helicopter shortly after takeoff and made an inquiry with the U.S. military, Onodera said.

Kanagawa Gov. Yuji Kuroiwa told reporters in Tokyo he was "very shocked about the frequency of accidents involving the U.S. military."

The Defense Ministry has called on U.S. military forces in Japan to provide more information on what happened and to take measures to prevent similar incidents.

In December, a 7.7-kilogram window fell off a CH-53E transport helicopter as it was flying over an elementary school just outside U.S. Marine Corps Air Station Futenma.

No one was injured, but the incident heightened concerns among people in Okinawa, which hosts the bulk of U.S. military facilities in Japan.

In February, a part from a U.S. military Osprey transport aircraft was found drifting near an islet in Okinawa, while an antenna fell off a U.S. F-15 fighter belonging to the U.S. Kadena Air Base in Okinawa.

A U.S. F-15 plane also crashed into the sea off Okinawa in June, with the pilot picked up by a search-and-rescue crew from Japan's Air Self-Defense Force.