A helicopter crash in the Alps left five people dead and a sixth in critical condition, French President Emmanuel Macron tweeted Tuesday.

"They take all the risks to save lives," Macron said. "This evening in Savoy three members of the French Air Rescue and two members of the Service Aerien Francais (SAF) were killed in a helicopter crash. Another injured survivor is fighting for their life. Support from the nation to the families, friends and colleagues of these French heroes."

Three other helicopters were dispatched on a rescue mission but were unable to reach the site of the accident due to fog. Nevertheless, a rescue team had managed to establish contact with the one surviving passenger.

Three helicopters and 40 personnel were involved in the rescue mission which carried on well into the evening.

Prime minister: 'A nation in mourning'

The local prefecture reported that the accident happened near the community of Bonvillard shortly after the pilot triggered an alarm.

Prime Minister Jean Castex also tweeted his support for the victims and said, in reference to the SAP employees: "They did not share the same uniform but they shared the same mission: saving lives. Their deaths leave a whole nation in mourning."

The search and rescue helicopter was carrying out a training mission when it went down at 1,800 meters (5,905 feet). The pilot was able to eject before alerting the local authorities to the incident.

The SAF is a private company specializing in medical evacuations and deployments in skiing areas. It is believed that the survivor is an employee of the company.

France's air accident bureau announced that it was opening an investigation into the accident and would send a team to the site.