The search, involving ships and planes from many nations, was part-suspended overnight, but aircraft with monitoring equipment continued to scour the sea.
The Air France Airbus was heading from Brazil to Paris when it disappeared.
France believes there is little hope of finding survivors from among the 228 passengers and crew aboard the flight.
Vessels from France, Spain, Senegal and Brazil are involved in the search, while the United States is said to be offering help with satellite reconnaissance.
Plane crews have narrowed their search to a zone of a few dozen nautical miles half-way between Brazil and west Africa, said Pierre-Henry Gourgeon, chief executive of Air France, late on Monday.
Their work may be aided by the Airbus's Argos beacons, which will emit signals for several days, he added.
An automatic report of a short circuit was the last communication received from the plane before it vanished over the ocean.
French officials believe it may have been disabled by a storm. French and US sources have ruled out terrorism as the cause of the plane's loss.
Most of the missing people are Brazilian or French but they include a total of 32 nationalities. Five Britons and three Irish citizens are among them.
French President Nicholas Sarkozy said he had told friends and relatives waiting at the Charles de Gaulle "the truth" - that the chances of finding anyone alive were "very small".
If no survivors are found, it will be the worst loss of life involving an Air France plane in the firm's 75-year history.
US spy technology
A French reconnaissance plane based in Dakar, Senegal, was due to reach the suspected crash area on Monday evening.
It was to be followed by two other French planes based in Dakar, and a naval vessel currently cruising in the Gulf of Guinea, several days' sailing away.
Spain and Senegal have also sent planes to help in the search.
Mr Sarkozy confirmed that his government was approaching Washington for help, following reports that France had asked the US for access to satellites and listening stations.
Brazil has sent out seven air force planes and three naval ships to help in the search, far off the north-eastern Brazilian coast.
Air force spokesman Col Jorge Amaral said earlier they were trying to reach the point from where the aircraft had last made contact, about 1,200km (745 miles) north-east of Natal.
But Maria Celina Rodrigues, the Brazilian consul in Paris, said the depth of the ocean would make it difficult for searchers.
"They are hoping they can find debris, pieces, lifejackets that eventually float, but that takes some time," she told the Associated Press.
The Brazilian authorities have also said they are investigating a possible sighting of wreckage in the area, reported Reuters news agency.
The crew of a TAM Linhas Aereas flight travelling over the area had reported seeing "bright spots" in the ocean about 1,300 km (800 miles) from the Fernando de Noronha archipelago off Brazil's north-eastern coast.
'Experienced' pilot
The plane's automatic report was generated at 0214 GMT on Monday, about four hours after Flight AF 447 left Rio de Janeiro, and as it was heading through turbulence towards the west African coast.
Missing man Arthur Coakley's wife, Patricia, and his business partner Ken Pearce
"A succession of a dozen technical messages" showed that "several electrical systems had broken down" which caused a "totally unprecedented situation in the plane", said Mr Gourgeon.
"It is probable that it was shortly after these messages that the impact in the Atlantic came," he told reporters at Charles de Gaulle airport, where the airliner had been due to land.
Flight AF 447 was flying at an altitude of 10,670m (35,000ft) shortly before it went missing.
A meteorologist who spoke to the Associated Press said tropical thunderstorms in the Atlantic could tower up to 15,240m (50,000ft).
"At the altitude it was flying, it's possible that the Air France plane flew directly into the most charged part of the storm - the top," said Henry Margusity, senior meteorologist for AccuWeather.com.
French officials have stressed that the plane's captain was very experienced, clocking up more than 11,000 hours of flight.
Crisis centres have been set up at Charles de Gaulle airport in Paris and Rio's Tom Jobim international airport.
