The city of Hamura in Tokyo protested to the U.S. military and the Japanese government Wednesday over the dropping of a U.S. Air Force parachute on the grounds of a school during a drill the previous day.

"It could have been a life-threatening incident" and was extremely regrettable, Hamura Mayor Shin Namiki said in letters handed to the U.S. military and the Japanese Defense Ministry by city officials, according to the city government. He also sent a letter of protest to the Foreign Ministry.

The U.S. military has told the Japanese government it will suspend parachute drills until the cause of the incident is known, Chief Cabinet Secretary Yoshihide Suga later said in a news conference.

While no one was injured in the incident, it rekindled safety concerns among local residents who are already uneasy about the recently announced deployment of accident-prone Osprey tilt-rotor aircraft at Yokota Air Base near the school.

The mayor said in the letters that such an accident "cannot be tolerated" from the perspective of securing citizens' safety and demanded the U.S. military stop descent training and implement thorough safety measures.

(Parachute found on school grounds in Tokyo. Photo courtesy of the city of Hamura)

Students found the parachute on tennis courts on the premises of the city-run junior high school Tuesday afternoon, according to city officials.

A number of incidents involving the U.S. military have occurred recently in Japan, including a metal-framed window falling from a CH-53E transport helicopter onto the playground of an elementary school adjacent to U.S. Marine Corps Air Station Futenma in Okinawa in December.