The French navy has located the flight recorders of the Air France jet that crashed in the Atlantic almost a year ago, but retrieving them may be an impossible mission, officials said.

The recorders have been localised with a margin of error of three nautical miles (five kilometres) in a remote area of the Atlantic Ocean.

But French military officials cautioned the breakthrough did not mean the black boxes will be successfully retrieved from the ocean floor.

"It's like trying to find a shoe box in an area the size of Paris, at a depth of 3,000 metres and in a terrain as rugged as the Alps," said navy spokesman Hugues du Plessis d'Argentre.

Flight 447 was en route to Paris from Rio de Janeiro when it went down on June 1 in stormy weather, killing all 228 people on board.

The crash was the worst in Air France's 75-year history.

Intensive but unsuccessful sweeps of the Atlantic Ocean using submarines equipped with deep-sea sonar had turned up some debris but no sign of the flight data and cockpit recorders.

The flight recorders from the Airbus A330 are key to understanding what caused the disaster, which remains largely unexplained.

The French navy started a new operation on Monday to find the black boxes.

Welcoming the news, the families of the crash victims said it raised hopes, but they quickly added that they would hold off on any celebration until the flight recorders were raised to the surface.

"This is a sign of hope. It's very good news for the families after 11 months of waiting," said Jean-Baptiste Audousset, president of an association grouping the families of 60 victims.

French government spokesman Luc Chatel added: "We must remain extremely cautious, because at this time we are talking about an area where they have been located."

"We then have to see if it is possible to recover the black boxes, depending on the depth, the surface area to cover... so I will remain extremely cautious," Mr Chatel told France Info radio.

Sonar breakthrough

The breakthrough in pinpointing the area where the black boxes can be found came after new computer software was used to decode data collected by deep-sea submarines during their search last summer, said the defence ministry.

The latest search effort was set to wrap up on May 25, but officials said it may be extended following the new findings.

The latest search covered an area of 1,500 square kilometres in a remote area far off the coast of Brazil.

The French air accident agency BEA has said the jet's speed probes, made by French firm Thales, gave false readings and were "one of the factors" in the crash but "not the sole cause".

Pilots' unions and some of the relatives of victims have accused Air France and Airbus of ignoring longstanding problems with air speed monitors on its jets in the run up to the disaster.

The companies insist their jets met all safety standards, but they have nevertheless now replaced the speed monitors with a different model produced by US firm Goodrich.

- AFP