Investigators made the first breakthrough yesterday in the probe into the crash of an airliner in the French Alps, as President Francois Hollande vowed everything would be done to explain the tragedy that killed 150 people.

Officials from the BEA crash investigation agency said they had extracted "usable data" from one of the Germanwings Airbus A320's two "black boxes" found among the debris, with recordings from the cockpit, but still had no explanation for the mysterious crash.

Hollande and German Chancellor Angela Merkel flew over the crash site to see the devastation for themselves before meeting rescue workers outside the crisis centre set up on Tuesday after the worst crash in France in four decades.

Spain's Prime Minister Mariano Rajoy also visited the centre to be briefed on the gruelling rescue operation in difficult mountain terrain where Flight 4U9525 crashed early Tuesday, scattering debris over a wide area.

Grieving relatives were also gathering near the crash site, where a counselling unit has been established.

The French president told reporters: "We have to understand what happened. We owe it to the families and the countries concerned by the drama."

"All light will be shed" on the mystery, he vowed.

Meanwhile, French investigation agency BEA released photos of the mangled black box, its metal casing torn and twisted by the violence of the impact.

BEA head Remi Jouty told reporters he had "not the slightest explanation" for the crash at this stage, although he revealed the plane was still flying when it smashed into the mountainside and did not explode mid-air.

Hollande said the casing of a second black box, which records technical flight data, had been found but not the device itself.

Authorities are still baffled as to why the plane suddenly began a fatal eight-minute descent shortly after reaching cruising altitude on its route between Barcelona and Duesseldorf.