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Brazilian air force Lt.-Col. Henry Munhoz holds a map of the search zone for crashed Air France Flight 447. ((Roberto Candia/Associated Press))

Sounds detected by search crews combing the Atlantic Ocean for the crashed Air France Flight 477 are not coming from the jetliner's black boxes, French officials said Tuesday, disputing reports in the country's local media.

French newspaper Le Monde had earlier reported that unmanned submarines had been placed in the water to search for the boxes after faint signals had been detected on Monday. The report did not cite its sources.

An aide to French Transport Minister Jean-Louis Borloo told The Associated Press that the "black boxes have not been detected."

The aide spoke on the condition of anonymity.

The aide confirmed that French military ships searching the area about 640 kilometres northeast of Brazil's Fernando de Noronha islands where the plane crashed have "heard sounds" in the ocean's depths but they are not believed to be coming from the flight's voice or data recorders.

French-chartered ships are trolling a search area with a radius of 80 kilometres, pulling U.S. navy underwater listening devices attached to 6,000 metres of cable.

French air accident authority BEA issued a statement Tuesday confirming the black boxes have not yet found and that sounds heard in the ocean are being investigated.

"In the context of the sea searches that are underway, work is undertaken on a regular basis that is aimed at eliminating any doubts related to any sounds that may be heard, and any findings will be made public," the statement said. But no sounds have yet been validated as coming from the black boxes, it said.

Last week, BEA director Paul-Louis Arslanian sternly warned against any unconfirmed leaks in the investigation, saying they could mislead the public and unnecessarily worry or encourage the families of victims.

The black boxes are seen as key to determining what happened to the Airbus 330 that disappeared from radar screens as it flew through stormy weather after leaving Rio de Janeiro en route to Paris.

The black boxes emit signals for about 30 days. The plane crashed on May 31 and the ocean floor where the debris is being collected has depths up to 7,000 metres.

All 228 people who were on the plane are believed to have died. Only 50 bodies have been recovered.

The investigation so far has focused on a flurry of automated messages sent by the plane minutes before it lost contact; one suggests external speed sensors had iced over, destabilizing the plane's control systems.

With files from The Associated Press