The United States will not fly its Osprey military aircraft again until it confirms their safety, a senior White House official said Thursday.

White House national security spokesman John Kirby said the United States will get to the bottom of last week's fatal crash of an Osprey off the coast of southwestern Japan during a routine exercise.

Kirby's remarks during a press briefing came a day after the U.S. military said it would ground its entire fleet of Ospreys deployed worldwide by the Air Force, Navy and Marine Corps, after finding that the Nov. 29 accident may have been caused by a malfunction.

He stressed that the stand-down is a "common procedure" when a serious aviation mishap occurs and there might be some larger systematic trouble.

White House national security spokesman John Kirby speaks at a press briefing at the White House on Dec. 7, 2023, in Washington (Getty/Kyodo)

The spokesman also said he believes that efforts to look into the cause of the accident will be carried out "expeditiously, but also as safely as possible."

The Air Force Special Operations Command said Wednesday the stand-down is intended "to mitigate risk while the investigation continues."

In Japan, a major U.S. treaty ally, concerns over the safety of the tilt-rotor aircraft, known for a number of mishaps and fatal crashes, have rekindled.

The crash off Yakushima Island, killing all eight on board the Air Force's CV-22 Osprey, is the deadliest involving the aircraft since their combat debut in 2007.

"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.

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