The cause of the explosion was not yet known, Marseille prosecutor Dominique Laurens said.
Eight people are not responding to calls and are thought to be under the rubble of two buildings that collapsed in an explosion early on Sunday in the southern French city of Marseille, local officials said.
The collapse caused a fire that has complicated rescue efforts and investigations, and it has not been brought under control, she told a news conference.
Television footage showed clouds of smoke rising from the rubble as firefighters tried to put out the fire, while trained dogs were used to try to locate victims.
"We have nothing, not even an ID card. We have lost everything," said a man who gave his name only as Roland, in an interview with local newspaper La Provence. He managed to get out of the building on 15 Rue de Tivoli with his wife and two children before it collapsed, together with a neighbouring building.
A third building partly collapsed.
Five people were taken to hospital with serious but not life threatening injuries.
Interior Minister Gerald Darmanin, who visited the site, said 30 buildings in the area were evacuated.
In 2018, around 1 kilometer (0.62 mile) from the tragedy, three buildings considered not fit for habitation collapsed, killing eight people.
The mayor of Marseille said a parallel could not be drawn, while the prosecutor said the buildings that collapsed on Sunday were not known to have any structural problems.
"Thoughts are with Marseille," President Emmanuel Macron said in a Twitter message.
