The French nuclear submarine Emeraude detected "acoustic signals" on Saturday, Morin told AFP, but this was not the first time that such noise had been picked up from the ocean depths.

"We are not able to confirm that these signals are coming from the black boxes of the plane," said Morin.

Le Monde newspaper reported on its website that the Nautile mini-submarine had been deployed on Monday after a "very weak signal" was detected several hundred kilometres off the coast of Brazil. No source was quoted in the report.

But the French bureau investigating the crash said the black boxes of the jetliner that crashed into the Atlantic on June 1 had not been found and that research teams "check out any sound" that might lead to them.

"Some signals have been detected but this is not the first time," said Paul Arslanian, the director of the BEA investigating agency.

"It does not mean that these signals are those from the black boxes. This requires confirmation and we have just started verification," he said.

Owned by Ifremer, the French oceans research institute, the Nautile is capable of operating at a depth of six kilometres (3.7 miles) and was used by research teams who explored the wreck of the Titanic.

It was taken to the debris zone on board the French maritime research vessel Pourquoi Pas and was to be deployed once the signal from the black boxes was detected.

The captain of the Pourquoi Pas, Philippe Guillemet, also formally denied that the black boxes had been located.

"We are detecting sound waves, but nothing has been confirmed, unfortunately. We pick up signals virtually every day. They need to be analysed," he told French radio Europe 1.

The Air France Airbus A330 carrying 228 people from Rio de Janeiro to Paris came down in the Atlantic on June 1. The cause of the disaster has not been established.

The homing beacons on the devices will only operate for around another week.

As well as the Pourquoi Pas, the French nuclear submarine Emeraude and two high-sea vessels equipped with sonars are taking part in the deep-sea search for the vital data and voice recorders of Flight 447.

The boxes -- which in fact are bright orange -- would help investigators piece together the final minutes of the ill-fated flight that went down during a storm as it was flying through turbulence.

No distress call was received from the pilots, but there was a series of 24 automated messages sent by the plane in the final minutes of the doomed flight.

Fifty bodies have been recovered from the crash zone, along with hundreds of pieces of the plane.

The remains of a British national and two Brazilians are among those recovered from the site, said the Interpol police organisation.

French investigators probing the crash have said that the airspeed sensors, or pitot probes, had been feeding inconsistent readings to the cockpit.

Conflicting airspeed data can cause the autopilot to shut down and in extreme cases lead the plane to stall or fly dangerously fast, possibly causing a high-altitude breakup.

But the BEA, along with Airbus and Air France, have said there is as yet no firm evidence linking the speed monitors and the crash of the jetliner.

Air France has upgraded all sensors on its long-haul fleet as a precautionary measure after protests from pilots.

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

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