The Ground Self-Defense Force said Thursday that it has been unable to determine the exact cause of the engine troubles that caused a deadly crash involving one of its helicopters off an island in the southern prefecture of Okinawa last April.

In its investigative report on the accident, which killed all 10 personnel on board, the GSDF described the nearly simultaneous engine failures as an "unprecedented event." However, it will fully resume operations of the same type of helicopter, although no date has been specified.

The force also pledged to take necessary measures to prevent any similar accidents from happening again, such as strengthening flight safety surveillance.