Hundreds of people are taking part in a rescue operation in the French Alps after a Germanwings airplane with 150 people on board went down near Barcelonette in Southern France while en route from Spain to D?sseldorf.

All 144 passengers and 6 crew are feared dead, and the EU Commission has declared three days of mourning over the incident.

The Airbus A320, flight 4U 9525, was operated by Germanwings, a low-cost subsidiary of German national airline Lufthansa. Both carriers were so far known for their impeccable safety record.

German Chancellor Angela Merkel, French President Francois Hollande and Spanish PM Mariano Rajoy are due to visit the site of the incident later on Wednesday. Merkel urged the media on Tuesday afternoon to avoid speculation as to what caused the incident.

The recovery operation has been resumed, and officials say it could last days as wreckage is spread across several square kilometers in a mountain ravine that is hard to reach.

"Everything is pulverised. The largest pieces of debris are the size of a small car. No-one can access the site from the ground," the BBC quotes Gilbert Sauvan, president of the general council of the Alpes-de-Haute-Provence as saying (that was where the crash took place).

On Tuesday evening, search teams recovered one of the "black boxes" of the aircraft, but authorities are warning a quick answer to what happened with flight 4U 9525 should not be expected.

The reasons for Tuesday's crash remain a mystery. Weather conditions in the French Alps region where the incident occurred were reported as good, and though the aircraft's descent was quick, its sink rate was not much beyond the average for landing approach and the situation did not amount to free fall.

On the other hand, initial reports suggesting a distress signal had been sent by the crew were later refuted, especially given the fact that the pilot in charge had ten years of experience.

Contact with French air traffic authorities was lost at 10:53 CET.

"This one is weird," David Learmount from the prominent Flight Global edition, noted on Twitter.

German-operated A320s do not crash in the cruise. Not these days. This one is weird. #Germanwings

— David Learmount (@DavidLearmount) March 24, 2015

Angel Borisov, a former Bulgarian aviation safety expert told the Bulgarian National Television on Wednesday that blaming the company would be wrong in this case, since the aircraft fleets of both low-cost and mainstream airlines undergo the same kinds of safety inspections.

The Airbus that crashed had been in use at Lufthansa since 1991, but its electronic system had recently been upgraded.