The crash site has presented investigators with huge challenges
The flight recorders from an Air France plane which crashed in the Atlantic last year have been located to within a 5km zone, a French official has said.
But Gen Christian Baptiste told AFP that retrieving the boxes from the ocean floor might be impossible.
All 228 people on board the flight from Rio to Paris died in the crash.
The cause remains unknown. The plane's airspeed probes had given false readings, but officials believe other factors must also have contributed.
Finding the boxes, which record flight data and cockpit conversations, should allow investigators to finally explain the mystery of why the plane came down.
It appears the location zone has been traced using images obtained during the first phase of searching for the wreckage, when the flight recorders were still emitting a signal.
French government and military officials have urged caution, saying there is no guarantee the flight recorders will be found.
"It's like trying to find a shoe box in an area the size of Paris, at a depth of 3,000m (9,800ft) and in a terrain as rugged as the Alps," French navy spokesman Hugues du Plessis d'Argentre told AFP.
A fresh search was launched earlier this year involving US and Norwegian ships with sonar probes and robots, in what officials described as "one of the most complex undersea operations ever".
The area covered by the high-tech vessels will now be reduced from 1,500 sq km (580 sq miles) to just 3-5 sq km (1-2 sq miles), in a remote area far off the coast of Brazil.
Air France flight AF447 from Rio de Janeiro to Paris disappeared on 1 June 2009 in stormy weather, killing all those on board.