Focus on debris
Colonel Jorge Amaral, from the Brazilian air force, said efforts would be focused on collecting the debris.
"We can't really say this is part of the airplane. The command centre needs to have at least one piece of the debris with a serial number to confirm that it belongs to the airplane," Amaral said.
Pilots from Brazil's largest airline said earlier that they had spotted what appeared to be fire in the Atlantic Ocean.
The Air France jet, which was flying from Brazil to France with 228 people on board, dropped off radar screens on Monday morning, hours after taking off from Rio de Janeiro.
Brazilian aircraft equipped with sensors are continuing to sweeping a part of the ocean where the airliner is thought to have disappeared.
Authorities said they are still trying to determine what caused the accident, but that they fear there is no chance of finding survivors.
Flight AF447 had encountered a severe thunderstorm before sending an automated message saying that its electrical system had failed at 02:14 GMT on Monday.
Pierre-Henry Gourgeon, Air France's chief executive, said "several electrical systems had broken down".
"It is probable that it was shortly after these messages that the impact in the Atlantic came," he said.
Jean-Louis Borloo, the French environment minister, said officials did not believe the Airbus A330-200 could have been brought down by something as simple as a bolt of lightning, which previous reports had suggested.
"There really had to be a succession of extraordinary events to be able to explain this situation," he told RTL radio.
|The jet lost radio contact over the Atlantic while flying from Rio de Janeiro to Paris
He said "the race against the clock has begun" to find the aircraft's two data recorders, which emit signals for up to 30 days.
Alain Bouillard, who led the probe into the crash of the Concorde in July 2000, has been put in charge of France's accident investigation team.
Herve Morin, France's defence minister, said "we have no signs so far" of terrorism, but all hypotheses must be studied.
Barack Obama, the US president, told French television stations the US is ready to do everything necessary to find out what happened.
The Pentagon said on Monday it had dispatched a surveillance aircraft and a search and rescue team to help Brazilian and French aircraft.
France has also asked Washington to use its spy satellites and listening posts to help with the search.
|Some evidence suggests lightning could have brought down the Airbus A330 jet [AFP]
A French military aircraft dispatched from Senegal, a west African country on the Atlantic seaboard, has also been searching the area.
The first navy ships are not expected to arrive in the search area until Wednesday.
Air France said the 216 passengers from 32 countries comprised 126 men, 82 women, seven children and a baby.
There were also 12 French crew members on board.
More than half of the passengers were either French or Brazilian.
If confirmed that all 228 people on board are dead, it would be the worst loss of life in Air France's history and civil aviation's worst accident for more than a decade.