French officials said on Wednesday it proves hopeless to find the Air France's black boxes on the ocean floor.

Officials said they may never discover why the aircraft crashed into the Atlantic Ocean killing 228 people on board.

France is dispatching a mini submarine that can explore to a depth of 6,000 meters and will try to locate the Airbus' flight data and voice recorders, which should shed light on a crash that has puzzled aviation experts.

Brazilian Air Force spokesman Colonel Jorge Amaral said on Wednesday that the newly spotted debris includes various objects spread across a 5-km area and a 20-km oil slick.

An aviation expert said the large distance between the wreckage zones might be an indication the plane broke up in the sky well before it hit the water.

Authorities are baffled by how a modern plane could have plunged out of the sky without giving its team of three experienced pilots time to send a mayday call.

French government minister Jean-Louis Borloo said the recorders were believed to be at a depth of between 3,660 meters and 3,700 meters, within reach of the submarine, but warned they would be hard to find.

We have never recovered black boxes that deep before and ... the sea currents are powerful that far down, he said.

Paul Louis Arslanian, the head of France's air accident investigation agency, said he was not sure that the black boxes would be recovered and said the probe might prove frustrating.

He also said the seabed in the area was rugged. It is practically in the middle of the Atlantic Ocean where the floor is very deep and varied. It is a mountainous seascape rather than a plain.

He said that the recorders are designed to send homing signals for up to 30 days when they hit water, but there is no guarantee they even survived the impact with the sea.