An Air France flight from Rio de Janeiro in Brazil to Paris is presumed to have crashed into the Atlantic early on Monday, killing all 228 people on board.
Most of the 216 passengers were Brazilian or French. Six Swiss citizens were reported on the flight.
The Swiss foreign ministry expressed its condolences and has set up a telephone information service for people who think relatives were on board. The number is 031 323 30 99.
The flight left Rio on Sunday evening local time, and was scheduled to arrive at 11.15am French time.
The final message from the plane, sent automatically, was received at 4.14am French time, and indicated an electrical fault.
French and Brazilian reconnaissance planes joined the search, and a French ship was diverted to help.
The head of communication at Air France said the plane, an Airbus A330, had probably been struck by lightning.
The plane had been in operation since 2005 and undergone a thorough service in April 2009. The crew was said to have been highly experienced.
If confirmed, the accident will have been the most costly in human life in the 75 years in which Air France has been in existence.
swissinfo.ch with agencies
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