FERNANDO DE NORONHA: Military planes and ships from Brazil and France homed in Wednesday on the bobbing wreckage of an Air France jet, as investigators tried to determine what brought the plane and its 228 passengers down in the middle of the Atlantic Ocean.

The planes stepped up overflights 400 miles (640 kilometres) northeast of the Fernando de Noronha islands off Brazil's northern coast, where an airplane seat, a fuel slick and pieces of white debris were spotted Tuesday in the vast ocean.

Rescue boats from several nations were sailing toward the site to start the recovery.

Air France Flight 447 vanished Sunday night about four hours into its flight from Rio de Janeiro to Paris. The reason for the crash remained unclear, with fierce thunderstorms, lightning or a catastrophic combination of causes as possible theories.

French accident investigators leading the investigation were holding a news conference later Wednesday.

"The nature of the debris, the concentration of the debris ... all combines to prove that the debris from Air France 447 has been found," French military spokesman Christophe Prazuck said Wednesday.

Brazilian Defense Minister Nelson Jobim told reporters in Rio that no bodies had been found and there was no sign of life.

If no survivors are found, it would be the deadliest crash in Air France's history, and the world's worst civil aviation disaster since the November 2001 crash of an American Airlines jetliner in the New York City borough of Queens that killed 265 people.

The floating debris is spread out in two areas about 35 miles (60 kilometers) apart, not far off the flight path of Flight 447, said Air Force spokesman Jorge Amaral.

In Paris on Wednesday, Prazuck said the French military was moving away from its sweeping aerial searches to "the next phase, the recovery of this debris, to be able to conduct the investigation and determine the probable zone of the accident, around which we must search for the black boxes." The naval recovery operation will start on the surface, Prazuck said, then could turn to the use of submarines to help find the black boxes.

The effort is expected to be exceedingly challenging. Storm season is starting in the zone and low visibility hampered rescue efforts Tuesday. Water depths in the area sink down to 22,950 feet (7,000 metres).

Gideon Ewers of the London-based International Federation of Air Line Pilots Association said bad weather and the plane's flight path could combine to spread the wreckage widely "The aircraft was cruising at 35,000 feet (10,500 metres)," Ewers said. "Wreckage could have dispersed over a wide area of ocean and then drifted even further apart while sinking to the ocean floor a couple of miles (kilometres) down." Remotely controlled submersible crafts will have to be used to recover wreckage settling so far beneath the ocean's surface. France dispatched a research ship equipped with unmanned submarines that can explore as deeply as 19,600 feet (6,000 metres).

A U.S. Navy P-3C Orion surveillance plane - which can fly low over the ocean for 12 hours at a time and has radar and sonar designed to track submarines underwater - and a French AWACS radar plane were joining the operation.

The cause of the crash will not be known until the black boxes are recovered - which could take days or weeks. They are meant to last 30 days underwater.

Even at great underwater pressure, the black boxes "can survive indefinitely almost," said Bill Voss, president and CEO of the Flight Safety Foundation in Alexandria, Virginia. "They're very rugged and sophisticated, virtually indestructible." Weather and aviation experts are focusing on the possibility of a collision with a brutal storm that sent winds of 100 mph (160 kph) straight into the airliner's path.

Towering Atlantic storms are common this time of year near the equator - an area known as the intertropical convergence zone. But several veteran pilots said it was extremely unlikely that Flight 447's crew intended to punch through a killer storm.

Since thunderstorms can rise to more than 60,000 feet high (18,000 metres), where passenger planes cannot climb over them, pilots will often weave left and right to find a route that avoids the worst of the weather.

"Nobody in their right mind would ever go through a thunderstorm," said Tim Meldahl, a captain for a major U.S. airline who has flown internationally for 26 years. "If they were trying to lace their way in and out of these things, they could have been caught by an updraft." The crew made no distress call before the crash, but the plane's system sent an automatic message just before it disappeared, reporting lost cabin pressure and electrical failure.

Brazilian officials described a three-mile strip of wreckage.

Jack Casey, an aviation safety consultant in Washington, D.C., and former accident investigator, said that it could indicate the Air France jetliner came apart before it hit the water.

On land, hundreds of relatives grieved deeply for those who were lost, a roster that included vacationers, business people, even an 11-year-old boy traveling alone back to school in England.

"We will miss your dancing feet," read a tribute from the Northern Ireland family of Eithne Walls, 29, a dancer-turned-doctor.

"We will miss your silliness, your wit and your hugs. We will always hold you in our hearts and you are never truly gone."