The Tokyo metropolitan government and several municipal authorities are seeking more information about the lifting of a ban on flights of Osprey tilt-rotor transport aircraft by the US military.

The US military had grounded its entire Osprey fleet deployed globally after one of the planes crashed into waters off the southwestern Japanese prefecture of Kagoshima in November. The aircraft belonged to the US Yokota Air Base in Tokyo and all eight crewmembers on board were killed.

The US forces on Friday announced the lifting of the ban.

A council comprising of Tokyo and municipalities surrounding the Yokota base delivered a document to the central government, asking it to clarify the cause of the accident. It also asked for details on measures to ensure safety and prevent a recurrence.

The document, addressed to the defense minister and others, points out that although the government says it will closely coordinate with the US side over the resumption of Osprey flights, an investigation into the accident is still ongoing.

It also says the US cited a material failure of a specific aircraft component as the cause of the accident, but what caused the failure and details of safety measures to be taken have not been disclosed.

The document says concerns are mounting among residents of areas around the US base about the safety of Osprey operations.