French investigators were on Wednesday probing what caused a mountain rescue helicopter to crash on a training mission in the Alps, leaving five dead and the sole survivor badly hurt.

The helicopter, an Airbus EC135 operated by a private firm, crashed Tuesday evening around the town of Bonvillard just outside Albertville, one of the main resorts in the French Alps.

The Savoy region prefect Pascal Bolot -- the top local official -- said among the six people on board only the flying instructor had survived but was badly wounded.

Initial reports said that it was the pilot who survived, but it was not clear if the flying instructor was himself at the controls of the helicopter at the time of the crash or another pilot.

Despite difficult weather conditions with fog at the crash site 1,800 metres (6,000 feet) above sea level, he was evacuated to hospital. Bolot said it was the flying instructor himself who had raised the alarm.

He added that an investigation has been opened by the prosecutor for Albertville but did not offer any explanation for how the crash happened.

Weather conditions were difficult at the time of the accident, with the local gendarmerie saying there was 40 centimetres (16 inches) of snow on the ground.

An Airbus EC135 helicopter belonging to Service aerien francais (SAF), the same model that crashed in the Alps Tuesday
An Airbus EC135 helicopter belonging to Service aerien francais (SAF), the same model that crashed in the Alps Tuesday AFP / JEAN-PHILIPPE KSIAZEK

French Interior Minister Gerald Darmanin had been due at the scene later Wednesday but his visit has now been delayed to Thursday. Investigators from France's Bureau of Enquiry and Analysis (BEA) agency for aviation are also set to arrive.

Two of those killed were members of a special mountain police unit for the Alps -- a captain aged 45 and a brigadier aged 39 -- who were taking part in a training exercise, France's national police force said on its Twitter account.

The other three killed were employees of the private firm that operated the helicopter and also employed the flying instructor.

"They'd take any risk to save lives," President Emmanuel Macron said in a tweet, paying tribute to those killed and the pilot "fighting for his life".

"They did not wear the same uniform but shared the same commitment -- to save lives," added Prime Minister Jean Castex. "Their passing leaves the whole nation in mourning," he added.

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