The Air Force Special Operations Command said in a statement the move was to "mitigate risk while the investigation continues" on the November 29 crash which killed eight US airmen.

The Naval Air Systems Command said later Wednesday it was following suit.

The hybrid V-22 Osprey aircraft, which can take off and land vertically like a helicopter and rotate its propellers forward to fly like an airplane, has been involved in three fatal accidents in just over a year.

The air force's decision was taken "to mitigate risk while the investigation continues" into what caused the fatal accident off southwest Japan, the Air Force statement said.

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

Grounding the Ospreys "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," it added.

Following the accident, Japan suspended flights of its own Ospreys and asked the US military to do the same on Japanese territory, as a precautionary measure.

Several recent accidents, in addition to the one off Yakushima Island, have highlighted questions about the reliability of the tilt-rotor aircraft, a joint project between aircraft manufacturer Boeing and helicopter specialist Bell.

At the end of August, three US Marines were killed in an Osprey accident in northern Australia, and in 2022, four others perished in Norway when their CV-22 crashed during a NATO training flight.