The cause of the explosion was not yet known, Marseille prosecutor Dominique Laurens 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, as reported by Reuters.
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.