The co-pilot of the doomed Germanwings flight appears to have "deliberately" crashed the plane after locking his captain out of the cockpit, French officials said Thursday, in revelations that sparked global shock.

In a chilling account of the final minutes of Germanwings Flight 4U 9525, lead prosecutor Brice Robin said 28-year-old German Andreas Lubitz initiated the plane's descent while alone at the controls.

Lubitz appeared to "show a desire to want to destroy" the plane, Robin told reporters after his team analysed the Airbus' cockpit flight recorder.

However, Lubitz is not believed to be part of a terrorist plot, officials said.

In an initial reaction, German Chancellor Angela Merkel said the findings added an "absolutely unimaginable dimension" to the tragedy in which 150 people were killed, mostly Germans and Spanish.

Prime Minister Mariano Rajoy of Spain said he was "deeply shaken" by the news and sent his "heartfelt affection" to the victims' families, dozens of whom had arrived near the crash site.

The passengers were killed "instantly" by the crash and were probably unaware of the impending disaster until the "very last moment," the French prosecutor said.

"The screams are heard only in the last instants before the impact."

"The co-pilot was alone at the controls," he said. "[He] deliberately refused to open the door of the cockpit to the pilot."

The pilot, believed to have gone to the toilet, made increasingly furious attempts to re-enter the cockpit, banging on the door, the recordings appear to show.

The co-pilot's motive remains a mystery, but investigators have all but ruled out terrorism.

"At this moment, there is no indication that this is an act of terrorism," Robin said.

Germany's interior minister echoed this, saying there was so far no indication of "a terrorist background."

But Robin was also unwilling to use the word "suicide."

"Usually when you commit suicide, you do it alone. When you're responsible for 150 people behind you, I don't necessarily call that suicide," he said.

Lubitz appeared to be conscious throughout, and Robin said it would be impossible to accidentally turn the descent button.

"If you passed out and leaned over on it, it would only go a quarter-way and do nothing. This action can only be deliberate," said the prosecutor, adding the co-pilot had set the controls to "accelerate the plane's descent."

A "stunned" Lufthansa Chief Executive Carsten Spohr said: "In our worst nightmares we could not have imagined that this kind of tragedy could happen to us here at the company."

He added no security "system in the world" could have prevented the co-pilot's actions.

Lubitz had passed all psychological tests required for training and underwent regular physical examinations, said the CEO.

In the first industry response to the disaster, low-cost carrier Norwegian Air Shuttle said it would require two people in the cockpit at all times.

Meanwhile, families and friends of victims gathered near the remote mountainous crash site area, where locals have opened their doors in a show of solidarity with the grieving relatives.

"We're all pitching in of course. There's no such thing as nationality, no such thing as religion," said one local volunteer, Charles Lanta.

Two planes arrived in southern France on Thursday from Barcelona and Duesseldorf with families and friends.

Tents were set up for them to give DNA samples to start the process of identifying the remains of loved ones, at least 51 of whom were Spaniards and at least 75 Germans.

Dozens of officials and police kept the waiting media at bay to ensure their privacy.

They were briefed by the prosecutor and reacted with "shock" to the findings, Brice said.

Meanwhile, the remains of their loved ones, found scattered across the scree-covered slopes, were being taken by helicopter to nearby Seyne-les-Alpes, a source close to the investigation told AFP.

Policeman Xavier Vialenc said his officers "were trying to gather everything they could" but the operation would be "long, very long, at least a fortnight."

The crash site, which is situated at about 1,500 meters altitude, is accessible only by helicopter or an arduous hike on foot.

French President Francois Hollande and Merkel flew over the site to see the devastation for themselves Wednesday.

It was the deadliest air crash on the French mainland since 1974 when a Turkish Airlines plane crashed, killing 346 people.

Lufthansa said the aircraft was carrying citizens of 18 countries. Three Americans and three Britons were confirmed among the victims.

Argentina, Australia, Belgium, Colombia, Denmark, Holland, Israel, Japan, Mexico and Morocco also had nationals on board.

The dead on board flight 4U 9525 included two babies and 16 German school exchange pupils. They were flying from Barcelona to Duesseldorf.

A second black box, which records flight data, has not yet been recovered. (AFP)