Floods washed away houses and destroyed roads and bridges surrounding the city of Nice on the French Riviera after almost a year’s average rainfall fell in less than 12 hours.
Nice Mayor Christian Estrosi said more than 100 homes had been destroyed or severely damaged.
Rescuers were also providing emergency assistance, including food and water, to residents living in isolated villages.
The missing people include two firefighters whose vehicle was carried away by a torrent when a road collapsed south of the village of Saint-Martin-Vesubie.
Authorities fear more victims as many families could not contact relatives due to mobile phone services being down.
French Prime Minister Jean Castex, who flew over the area in a helicopter, expressed “grave concern” over the toll of the flooding.
About 10,500 homes were left without electricity on Sunday, French energy company Enedis said.
In Italy, a firefighter was killed during a rescue operation in the mountainous northern region of Val d’Aosta.
A search team also found a body in the Piedmont region’s Vercelli province, where a man had been swept away by floodwater.
Italian firefighters also rescued 25 people trapped on the French side of a high mountain pass due to the flooding.
