An Air France plane with 228 people on board was presumed to have crashed into the Atlantic Ocean on Monday after hitting stormy weather during a flight from Rio de Janeiro to Paris.
The airline offered its condolences to the families of the passengers, making clear it did not expect any rescue.
"It's a tragic accident. The chances of finding survivors are tiny," French President Nicolas Sarkozy said at Paris's Charles de Gaulle airport after meeting passenger relatives.
The plane was packed with 216 passengers including seven children and one baby, Air France said. Most of them were French or Brazilian but they included around 20 Germans and several other nationalities. Twelve crew members were also on board.
The full Airbus jet flew into storms and heavy turbulence four hours after take-off from Rio and 15 minutes later sent an automatic message reporting electrical faults, the airline said.
There was no sign that the crew had sent a mayday message or any indication that signal-emitting emergency locators had activated on impact as is normally the case in crashes.
A company spokesman said several of the plane's mechanisms had malfunctioned.
"It is probably a combination of circumstances that could have led to the crash," he said, adding that the airliner might have been hit by lightning.
Aviation experts said lightning strikes on planes were common and could not alone explain a disaster.
Senior French minister Jean-Louis Borloo ruled out a hijacking, saying the plane would have landed somewhere, but said it was too early to exclude any other scenario.
The Brazilian air force said the plane was far out over the the sea when it went missing.
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.
International search effort
Military planes took off from the island of Fernando de Noronha off Brazil's northeast coast to look for it and the Brazilian navy sent three ships to help in the search.
France sent one of its air force planes from west Africa and several ships. Sarkozy said Spain was helping in the mission and Paris had asked the United States to assist in locating the crash site using U.S. satellite data.
"It seems the zone has been identified down to within 10 nautical miles," Borloo said on France 2 state television.
The plane left Rio de Janeiro on Sunday at 2200 GMT.
On its flight northeast from Rio, the aircraft would have had to pass through a notorious storm patch shifting around the equator known as the Intertropical Convergence Zone.
"It is a zone in the tropics where you can have particularly deep thunder clouds," said Barry Gromett, a meteorologist at the London Weather Centre.
Tearful relatives were led away by airport staff in Paris to a private area where psychologists were ready to assist them.
Executives from French tyre company Michelin and from the Brazil unit of German steelmaker ThyssenKrupp were among the passengers, the companies said.
The plane was an Airbus 330-200 powered with General Electric engines. If the plane is confirmed to have crashed, it would be the first time an A330 has been lost during an operational airline flight.
Air France said the plane had 18,870 flight hours on the clock and went into service in April 2005. It last underwent maintenance in a hangar in April this year. The pilots were also very experienced, the airline said.
The last incident with major loss of life involving an Air France plane was in July 2000 when one of its Concorde supersonic airliners crashed just after taking off from Paris, bound for New York.
At least 113 people died in the disaster.
(Additional reporting by Clements Guillou, Jean-Baptiste Vey, Gerard Bon, Astrid Wendlandt and Tim Hepher in Paris, Pedro Fonseca in Rio; editing by Crispian Balmer and Richard Meares)