The defense ministry said the boost had come through detailed follow-up analysis of sonar readings taken in the first few weeks after an Air France jet crashed into the Atlantic killing 228 people on June 1 last year.
Finding the black boxes is seen as essential to help crash experts and relatives understand exactly what caused flight 447 to plunge into a remote part of the Atlantic during an equatorial storm on its journey from Rio de Janeiro to Paris.
"It is probably the signal (of the boxes)," said General Christian Baptiste, deputy spokesman at the Defense Ministry.
The discovery of a possible 'ping' from at least one of the recorders on board the Airbus A330 has allowed experts to narrow the search to a few square kilometers from several thousand ahead of the anniversary of the airline's worst crash.
"Does this mean we have found the black boxes? We are still far from certain," Baptiste said. "The search zone still equates to an area the size of Paris and we have to find an object the size of a shoebox in seabed terrain which looks like the Andes," he told a news conference.
The recording which could contain the signal emitted from the recorder devices, buried until now behind background noise, was made on July 1, exactly one month after the crash.
Black box flight recorders are designed to emit homing signals for around 30 days.
"It is possible this will help us find important pieces of wreckage, and if we have a lot of luck a black box could be found in one of these pieces of wreckage," Baptiste said.
Two sophisticated salvage vessels, using miniature submarines, have been scouring a 3,000-square-km area to try to locate the flight recorders of the Airbus A330 plane.
Baptiste said the search was now about 400 kilometers northwest of Brazilian islands of Sao Pedro and Sao Paulo.
Air France said the breakthrough was "excellent news.”
Recovery will depend on the depth where wreckage could lie, anywhere between 1 km and 4 km down.
A spokeswoman at France's air accident investigation authority BEA said Air France and Airbus had spent 13 million euros on the inquiry and French state bodies 15 million euros.
Speculation about the cause of the crash has focused on possible icing of the aircraft's speed sensors, which appeared to give inconsistent readings seconds before the plane vanished.
