WASHINGTON -- The US military said Wednesday that it is grounding its entire fleet of V-22 Osprey aircraft one week after an Air Force CV-22 crashed off the coast of Japan, killing eight US airmen onboard.

The US Air Force Special Operations Command (AFSOC) said in a statement that its commander, Tony Bauernfeind, directed an operational standdown to "mitigate risk while the investigation continues" into the Nov 29 crash of a CV-22 Osprey off the coast of Yakushima, an island in southern Japan, that killed all the eight crew members, all of them servicemen of AFSOC.

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

The remains of six airmen have now been recovered, with the two others still unaccounted for, AFSOC said in a statement on Tuesday.

The search and rescue operations have transitioned to search and recovery operations, meaning it is believed no survivors can be found.

Following AFSOC's decision, the Naval Air Systems Command (NAVAIR) said in a separate announcement that it is also grounding all of its Ospreys.

NAVAIR is responsible for the variations of Osprey flown by the Navy and the Marine Corps.

Variants of the V-22 Osprey are used by several services -- the MV-22 by the Marines, CV-22 by the Air Force, and CMV-22 by the Navy, with some 400, 51 and 27 Ospreys respectively, according to US media.