This June 17, 2013 photo shows a US Bell Boeing V-22 Osprey aircraft. (PHOTO / AP)

WASHINGTON/TOKYO - The US military said on Wednesday it was grounding its fleet of V-22 Osprey aircraft after a fatal crash last week off the coast of Japan that killed eight people onboard.

Tokyo had already grounded its small fleet of the tilt-rotor aircraft, whose deployment in the country has been controversial, with critics saying the Boeing and Bell Helicopter developed Osprey is prone to accidents.

"Preliminary investigation information indicates a potential materiel failure caused the mishap, but the underlying cause of the failure is unknown at this time," US Air Force Special Operations Command (AFSOC) said in a statement.

The deployment of the aircraft in pacifist Japan has faced opposition, especially among residents of the country's southwest Okinawan islands, where there has been a large US military presence since Japan's defeat in World War Two

The latest crash happened during a routine training mission last week off Yakushima Island, about 1,040 km southwest of the capital, Tokyo.

At least 400 of the transport aircraft have been delivered and are mainly used by the US Air Force, Marines and Navy, according to Boeing.

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Immediately after the crash, Japan grounded its fleet and asked the US to suspend flights of its V-22s operating in the country. The US initially said it was suspending flights from the doomed aircraft's unit but said other Ospreys would continue to fly after safety checks.

It is not unusual for the US military to ground entire fleets after fatal accidents. A spokesperson for Japan's defence ministry said on Thursday its aircraft remained grounded.

"It goes without saying that ensuring flight safety is the highest priority in the operation of aircraft," Japan's chief cabinet secretary, Hirokazu Matsuno, said on Thursday. "We will continue to request information sharing with the US side to ensure flight safety."

The deployment of the aircraft in pacifist Japan has faced opposition, especially among residents of the country's southwest Okinawan islands, where there has been a large US military presence since Japan's defeat in World War Two.

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An Osprey crash there in 2016 also led the US to ground its fleet of the aircraft in Japan.

According to the Flight Safety Foundation, at least 50 personnel have died in crashes operating or testing the aircraft. More than 20 of those deaths came after the V-22 entered service in 2007.

In August, three US Marines died in an Osprey crash off the coast of northern Australia while transporting troops during a routine military exercise.

In 2022, four US personnel were killed when an Osprey crashed in a remote part of northern Norway during a NATO training exercise.