France said on Monday the pilots of the Air Algérie passenger plane that crashed in Mali, killing all 118 people on board, had asked to turn back, in a new development to a tough probe into the tragedy

"What we know for sure is that the weather was bad that night, that the plane crew had asked to change route then to turn back before all contact was lost," Foreign Minister Laurent Fabius told reporters in his latest briefing about Thursday's disaster.

It had previously been known that the crew asked to change route due to bad weather conditions, but the revelation they then demanded to turn back is a new development.

Speaking hours after the black box flight recorders of the McDonnell Douglas 83 jet arrived in Paris from Mali to help investigators, Fabius added air crash experts currently on the remote desert site of the accident were toiling away in "extremely difficult conditions".

France's transport minister, meanwhile, warned that analyzing the crucial black boxes that record flight data and cockpit conversations could take "weeks".

'Heat is overwhelming' 

Fabius said that more than 20 air accident experts were currently in Mali's remote, barren Gossi area where the plane came down, working in tough conditions to determine why the plane plunged into the ground and to try and recover remains of the victims.

"The (human) remains are pulverized, the heat is overwhelming with rain to boot and with extreme difficulties in communicating and in transport," he said at the foreign ministry, where the flag flew at half-mast in mourning for the tragedy that saw entire families wiped out.

Video footage of the Gossi area showed a scene of devastation littered with twisted and burnt fragments of the plane.

France bore the brunt of the tragedy, with 54 of its nationals killed in the crash of flight AH5017, which had taken off from Ouagadougou in Burkina Faso bound for Algiers.

On Monday, flags on government buildings across the country flew at half-mast in mourning for the victims, who also hailed from Burkina Faso, Lebanon, Algeria, Spain, Canada, Germany and Luxembourg.

Several towns across France that lost entire families or couples to the tragedy also announced they would pay hommage to people they held dear.

The central village of Menet, where a family-of-four perished in the crash, said a silent march would take place Friday in front of the places where the victims used to go, such as the school or certain shops.

"People in the village can't quite realise what happened. For us, the footage we see on television is extremely violent," said the mayor Alexis Monier.

Fourth crisis meeting in France

In Paris, President Francois Hollande held another crisis meeting Monday morning with ministers at the presidency.

Paris has taken the lead in the probe, and Hollande has said the remains of all passengers on the plane — not just French nationals — would be repatriated to France.

The accident is the worst air tragedy to hit France since the crash of the Air France A330 from Rio de Janeiro to Paris in June 2009.

It was also the third crash worldwide in the space of just eight days, capping a disastrous week for the aviation industry.

On July 17, a Malaysia Airlines plane was shot down in restive eastern Ukraine, killing all 298 people on board.

And a Taiwanese aircraft crashed in torrential rain in Taiwan on Wednesday, killing 48.

Apart from air accident experts, France has also dispatched military forces already stationed in Mali since its offensive last year to free the country's north from the grip of Islamists and Tuareg rebels.

Fabius said that by Monday evening, a total of 200 French forces were due to have arrived on site, as well as Malian soldiers and Dutch forces from the MINUSMA UN stabilisation force in Mali.

"The site has been secured," he said.