The U.S. military said Wednesday it has temporarily grounded its entire fleet of Osprey aircraft after finding that last week's deadly crash off the coast of southwestern Japan during a routine exercise may have been due to a malfunction.

The decision to ground all Ospreys deployed worldwide by the Air Force, Navy and Marine Corps came after the Japanese government repeatedly asked the United States to suspend flights of the aircraft in Japan amid renewed concerns over their safety.

The stand-down is intended "to mitigate risk while the investigation continues" into the Nov. 29 accident, the Air Force Special Operations Command said. The crash off Yakushima Island, killing all eight on board the Air Force's CV-22 Osprey, is the deadliest involving the tilt-rotor aircraft since their combat debut in 2007.

File photo taken in September 2018 shows a CV-22 Osprey aircraft at the U.S. military's Yokota Air Base in the western suburbs of Tokyo. (Kyodo) 

"Preliminary investigation information indicates a potential materiel failure caused the mishap, but the underlying cause of the failure is unknown at this time," the command said.

It said the stand-down will provide "time and space for a thorough investigation to determine causal factors and recommendations to ensure the Air Force CV-22 fleet returns to flight operations."

The Naval Air Systems Command, which is in charge of the Marine Corps and Navy Osprey variants, separately said it has grounded all the aircraft to implement more risk mitigation controls.

In Tokyo, Japan's top government spokesman Hirokazu Matsuno said at a press conference, "It goes without saying that ensuring flight safety is the highest priority."

After the CV-22 aircraft, assigned to the 353rd Special Operations Wing at Yokota Air Base in the western suburbs of Tokyo, disappeared from radar and went down off the island, Japan grounded its forces' own fleet of 14 Ospreys.

Chief Cabinet Secretary Hirokazu Matsuno holds a press conference in Tokyo on Dec. 7, 2023. (Kyodo)

Ospreys have a history of mishaps, including fatal accidents. In late August, a U.S. Marine Corps Osprey crashed on a remote Australian island during a joint exercise with countries including the Philippines, killing three of the 23 who were aboard the aircraft.

Until the announcement on Wednesday, U.S. military flights involving Osprey variants other than the one that crashed had not been completely suspended in Japan.

The announcement was made a day after U.S. President Joe Biden, as well as the air force command, released a statement saying all the eight service members, including pilots and flight engineers, who were in their 20s and 30s, are now considered dead.

The U.S. military's search and rescue operations were switched to search and recovery mode.

In Tokyo on Thursday, Japanese Defense Minister Minoru Kihara, speaking at the lower house committee on national security, said Japan will urge the United States to get to the bottom of the latest Osprey accident.

Kihara also said he believes the United States will share with Japan "as much detailed information as possible."

Related coverage:

3 more bodies of crashed U.S. Osprey crew members recovered in Japan

Biden "heartbroken" over loss of 8 in U.S. military Osprey crash in Japan