The flaw that led to the loss of tail rotor control was unique to the aircraft involved and would not prevent other CH-53D Sea Stallion transport helicopters from going on an upcoming mission to Iraq, the military said in a statement, adding the findings were preliminary.

Six CH-53Ds took off from Okinawa Sunday, despite Tokyo's insistence the craft be grounded until preventive measures were taken and an official explanation given.

"We expressed our regret and we reiterated that the helicopters should be grounded until the full cause of the accident is found and the necessary precautions taken," said foreign ministry spokesman Hatsuhisa Takashima.

A protest was delivered by a senior Japanese official to an acting US ambassador Sunday, he said.

No civilians were hurt, but citizens staged protests after the aircraft swiped a campus building as it crashed onto the grounds of Okinawa International University in Ginowan, some 1,600 kilometers (1,000 miles) southwest of Tokyo on August 13.

Three marines were injured in the accident.

"A small retaining device in a subcomponent of the tail rotor assembly was missing, leading to a loss of tail rotor control," the Marine Corps Base at Camp S.D. Butler in Japan's southern Okinawa said in a statement.

"The cause was solely unique to the CH-53D involved in the accident," the statement said.

All CH-53Ds had been grounded and checked following the incident.

Around 44,600 US troops are based in Japan, two-thirds of whom are stationed in Okinawa.

Prime Minister Junichiro Koizumi vowed in June to make "sincere efforts" to reduce the "heavy" burden on Okinawans from having US military facilities close by, made worse by a series of crimes committed by US soldiers.

WAR.WIRE