France's transport minister said there were "no survivors" from the crash of the Germanwings Airbus A320.
"The circumstances of the accident, that are not yet clear, make us think there are not thought to be any survivors," President Francois Hollande told reporters.
The plane was travelling from the Spanish coastal city of Barcelona to the German city of Duesseldorf when it went down in the Barcelonnette area in southeastern France.
The airliner, which was carrying 144 passengers and six crew, issued a distress call at 10:47 am (0947 GMT), civil aviation authorities said.
"The debris of a passenger plane has been found in the region of Barcelonette," said interior ministry spokesman Pierre-Henry Brandet.
He said "major rescue efforts" had been mobilised.
The plane belonged to Germanwings, a low-cost affiliate of German airline Lufthansa based in Cologne which had no fatal accidents.
The single-aisle A320 typically seats 150 to 180 people.
Crisis cell
Interior Minister Bernard Cazeneuve immediately headed to the scene while Prime Minister Manuel Valls said he had called an inter-ministerial crisis cell.
"We don't know the reasons for the crash, we clearly fear that the 142-150 passengers and personnel have been killed considering the circumstances of the crash," said Valls.
"All is being done to understand what happened and to help the families of the victims," he said.
Germanwings had no immediate comment on what caused the crash.
"We have recently become aware of media reports speculating on an incident though we still do not have any confirmed information," it said in a statement.
"As soon as definite information is available, we shall inform the media immediately," it said, urging people to monitor its website www.germanwings.com.
A spokesman for Airbus, the European aerospace giant, said it could not make any comment "for the moment".
PHOTO GALLERY: Germanwings – The crash site
PHOTO GALLERY: Germanwings plane crashes in Southern French Alps
"We have no information on the circumstances of the accident," the spokesman told AFP, adding that the company had opened a "crisis cell".
In July 2000, an Air France Concorde crashed shortly after take-off from Paris's Charles de Gaulle airport en route for New York, leaving 113 people, mainly Germans dead and eventually leading to the supersonic airliner being taken out of service.
The world's worst air disasters remain the March 27, 1977, collision of two Boeing 747s on the runway at Tenerife in the Canary Islands, killing 583 people, and the August 12, 1985 crash into a mountainside of a Boeing 747 belonging to Japan Airlines, killing 520 people.
France's leading air traffic controller union SNCTA called off a strike planned from Wednesday to Friday after news of the crash.
"We are suspending our planned strike as a result of the emotions created in the control rooms by the crash, particularly in Aix-en-Provence," the union's spokesman Roger Rousseau told AFP.
France's leading air traffic controller union SNCTA has called off a strike planned from Wednesday to Friday.
"We are suspending our planned strike as a result of the emotions created in the control rooms by the crash, particularly in Aix-en-Provence," the union's spokesman Roger Rousseau told AFP.
Lufthansa itself was hit by a four-day pilots' strike last week, although this did not affect Germanwings.
Shares in Airbus slumped on news of the accident, down 1.77 percent to 58.94 euros at 1100 GMT after briefly sliding two percent.
