Seyne-les-Alpes: A pilot of Germanwings Flight 4U 9525 may have been locked out of the cockpit, minutes before its crash into the French Alps, an audio file found in the battered remains of the cockpit voice recorder suggests.

The New York Times has reported a "senior military official involved in the investigation" describing a "very smooth, very cool" conversation between the two pilots during the first part of the flight from Barcelona to Dusseldorf.

Then one of the pilots left the cockpit, the door closed behind him and he could not re-enter.

"The guy outside is knocking lightly on the door, and there is no answer," the investigator told the Times. "And then he hits the door stronger, and no answer. There is never an answer.

"You can hear he is trying to smash the door down."

It was unclear why the pilot left the cockpit, but by the end of the flight he had not been able to get back in.

After 9/11, Airbus installed new features on its cockpit doors to protect the crew, including a reinforced structure, and a toggle control in the cockpit to stop the door from opening from the outside.

Earlier, crash investigators had said the smooth curve of the plane's descent was "compatible" with human control, rather than an uncontrolled fall.

One hundred and fifty people died in the crash, including a mother and son from Victoria, and a group of schoolchildren from Germany.

On Wednesday, French crash investigation body BEA announced it had extracted the recording from the mangled and broken remnants of the recorder, a bright orange box found on Tuesday amid the debris scattered down a steep, dark ravine near Seyne-les-Alpes.

Remi Jouty, head of BEA, said there were "some problems" reading the data on the cockpit recorder, which holds audio from four microphones and captures conversations and ambient noise during the flight.

"Nevertheless it was good news and a relief for us that we were able to extract an audio file that we can use," he said at a press conference in Paris.

Mr Jouty did not say what, or who was audible on the recording.

He said the debris field left by the crash was relatively small, suggesting the plane had not exploded in mid-air, but disintegrated on impact.

The curve of the plane's descent suggested some element of control, Mr Jouty said.

"The curve is compatible with an aircraft controlled by pilots, except for the fact that we can't imagine pilots sending an aircraft into a mountain, but it may also be compatible with an autopilot," he said.

He said the final communication from the plane was a routine message about permission to continue on its route.

Lufthansa CEO Carsten Spohr said the plane had appeared in "perfect technical shape" in its most recent inspections.

However some media reported the plane was a year away from needing a thorough 'lifespan-extending' refit.

According to some reports, parts of the plane's second data recorder's container have been recovered, but not the part containing the data itself.

All Wednesday, rescue and investigation teams flew in and out of the picturesque village of Seyne-les-Alpes, about 10 kilometres from the crash site, where the emergency effort has its headquarters.

More than 600 rescue workers and aviation investigators were in the area, French officials said. However much fewer – about 50 –  have so far been able to reach the crash site, scaling down shale rocks from above, or being winched from helicopters.

As well as recovering the data recorders, the priority is to recover the victims' remains – and to protect them from the elements before they can be recovered. Due to the difficult terrain and the condition of the bodies, officials told the media, it could be weeks before the last remains are recovered.

One gendarme said most of the bodies had been broken up by the speed of the impact, and human remains were being marked with white flags.

Weather is also a potential hazard - there are signs of spring in the air, and though the highest peaks are still snow-capped, the ice is melting and it rains more often than it snows.

Gendarmes have cordoned off many of the tracks into the region around the crash.

Residents of Seyne -  a tourist town used to welcoming visitors, though never in such tragic circumstances - rushed to help the emergency workers, advising on the best way over the mountain to the crash site.

More than 200 people in the region have offered their homes to house the families of the victims, and a team of psychologists, carers and local teachers acting as interpreters have assembled to help them when they arrive.

A temporary funeral parlour has been set up in Seyne.

Some relatives are on their way to the Alps, flying from Germany and Spain with Lufthansa, owner of Germanwings, paying for the flights. One group of 14 relatives of Spanish victims was coming by bus, having indicated they did not want to fly.

Relatives are providing DNA samples to help with victim identification.

French President Francois Hollande and German Chancellor Angela Merkel flew by helicopter over the crash site on Wednesday before landing in Seyne.

They were joined by Spanish Prime Minister Mariano Rajoy at the small village of Le Vernet, in the shadow of the mountain range where the plane crashed.

A small room next to a restaurant has been converted into a community chapel. Inside a makeshift memorial flame burns, and a simple bouquet of flowers sits by a condolence book. Outside, Spanish, French and German flags stand as a mark of respect.

It is expected that family members will visit the chapel, as a place to pray for their loved ones – and perhaps the closest they will be able to get to the actual site they lost their lives.

Afterwards, at a press conference in Seyne, Ms Merkel said the leaders were "very much united and connected".

She said relatives and friends of the victims would be very welcome there if they wished to come to where the tragedy happened.

Mr Rajoy said the leaders were "together with you in your pain".

"The worst that could happen to a human being has happened," he said.

Mr Hollande praised the rescue workers for securing the site and protecting the bodies of the dead "in a difficult area", as well as protecting parts of the destroyed aircraft to preserve them for the crash investigation.

"In times of trial, and we've had a few here in recent months, there's also solidarity," he said. "Human solidarity is among us. And I'm proud that France can give this image in this moment of pain."

More names have emerged of those lost in the crash, though the airline has not yet released a definite list. They include Australians Greig Friday, 29, and Carol Friday, 68, a mother and son from Victoria, 16 students and two teachers from a school in Haltern, Germany, and others from Britain, the US, Argentina, Colombia, Venezuela, the Netherlands, Mexico, Japan, Denmark, Israel and Iran. Of the 144 passengers, 72 were German and 51 Spanish.

Thomas Winkelmann, CEO of Germanwings, said the list was not yet complete because the airline is trying to contact relatives of 27 victims. Some victims held dual citizenship.

Australian consular officials have arrived in Digne-les-Bains near Seyne, where the French authorities have established a Crisis Centre. They are liaising with French authorities on the recovery effort, including identification of remains, Fairfax understands.

The airline has not yet revealed the names of the pilots, beyond saying the captain had more than 10 years' experience, including 6000 flight hours on Airbus models.