An Airbus operated by Lufthansa's Germanwings budget airline crashed in a remote snowy area of the French Alps on Tuesday and all 150 on board were feared dead.
French President Francois Hollande said he believed none of those on board the A320 had survived, while the head of Lufthansa spoke of a dark day for the German airline.
Germanwings confirmed its flight 4U9525 from Barcelona to Duesseldorf crashed in the French Alps with 144 passengers and six crew members on board.
Hollande said: "The conditions of the accident, which have not yet been clarified, lead us to think there are no survivors."
Officials said the plane issued a distress call at 0947 GMT, about 52 minutes after take-off.
Unofficial website tracking data suggested the aircraft made a sharp descent from its cruising height of 35,000 feet but that it did not appear to have plummeted as quickly as aircraft known to have lost complete control.
However, safety experts warned against reading too much into the third-party data, especially over remote areas, and said black boxes holding the probable answers to the crash were expected to be retrieved quickly.
The accident happened in an alpine region known for skiing, hiking and rafting, but which is hard for rescue services to reach. As helicopters and emergency vehicles assembled, the weather was reported to be closing in.
"There will be a lot of cloud cover this afternoon, with local storms, snow above 1,800 metres and relatively low clouds. That will not help the helicopters in their work," an official from the local weather centre told Reuters
Hollande said there were likely to be significant numbers of Germans on the flight. Spain's deputy prime minister said 45 passengers had Spanish names.
It was the first crash of a large passenger jet on French soil since the Concorde disaster just outside Paris nearly 15 years ago. The A320 is a workhorse of worldwide aviation fleets. They are the world's most used passenger jets and have a good though not unblemished safety record.
Lufthansa Chief Executive Carsten Spohr, who planned to go to the crash site, spoke of a "dark day" for the airline.
"We do not yet know what has happened to flight 4U9525. My deepest sympathy goes to the families and friends of our passengers and crew," Lufthansa said on Twitter, citing Spohr.
"If our fears are confirmed, this is a dark day for Lufthansa. We hope to find survivors," it said.
A spokesman for France's DGAC aviation authority said the airliner crashed near the town of Barcelonnette about 100 km (65 miles) north of the French Riviera city of Nice.
French and German accident investigators were heading for the crash site in Meolans-Revel, a remote and sparsely inhabited commune in the Alps.
, Lufthansa Chief Executive, reacts as he makes a statement at the Frankfurt airport March 24, 2015. Photo: Reuters/Kai Pfaffenbach
German Transport Minister Alexander Dobrindt would also travel to the crash site, a ministry spokesman said.
Airbus said it was aware of reports of the crash.
"We are aware of the media reports," Airbus said on Twitter. "All efforts are now going towards assessing the situation. We will provide further information as soon as available."
Officials at Barcelona airport said the flight took off at 0855 GMT. Families of those on board the plane were gathering at a specially prepared building there.
The crashed A320 is 24 years old -- at the upper end of useful life of an aircraft in first-tier airlines -- and has been with the parent Lufthansa group since 1991, according to online database airfleets.net