Japan's Defense Minister says flight recorder analysis indicates that a mid-air collision was the cause of last month's crash of two Maritime Self-Defense Force helicopters.

The two SH-60K patrol helicopters crashed off Torishima Island in the Izu Islands, south of Tokyo, during nighttime drills on April 20. One of the eight crewmembers was confirmed dead. The seven others have yet to be found.

The MSDF had said it is highly likely the choppers collided in mid-air.

Defense Minister Kihara Minoru told reporters on Thursday that primary analysis of the flight recorders shows there were no abnormalities with the aircraft and their performance before the crash.

Kihara revealed that a sudden big impact was simultaneously recorded on both helicopters' devices at the same location, and this indicates the two choppers collided and went down.

He added that further analysis is needed to determine the precise cause of the collision.

Kihara also announced that training flights involving a single SH-60K will resume on Friday or later. Flight training with these helicopters has been suspended since last month's accident.