The first victim - a man - was pulled from the wreckage Tuesday, prosecutor Xavier Tarabeux said, adding that he needed to be identified.
Authorities said they were looking into what caused the collapse of the buildings, described by residents of the area as dilapidated and in need of fix.
Two buildings containing apartments at numbers 63 and 65 Rue de l'Aubagne suddenly collapsed in the rundown Noailles district of the Mediterranean port on Monday morning.
Mr Castaner said: "During the first clearing operations we've found some pockets of air that means we still have some hope of finding and identifying a survivor".More news: Trump points to second term as U.S. president
"Everybody knew about the problems with the two collapsed buildings", said Patrick Lacoste, a spokesman for a local housing action group.
Residents said Tuesday the structural risks of the buildings and others like them were widely known, but that city officials did little when alerted about them. "People died for nothing even though we knew", he added.
People had been living in nine of the 10 apartments at number 65, while a shop occupied the ground floor.
A young waiter watched the scene with tears in his eyes, anxious for news of an Italian woman who lived in the building. The others were so dilapidated they had been condemned and were boarded up, though locals said they were frequently used by squatters.More news: Red Star upsets Liverpool in Champions League
One of those thought dead includes a mother who did not collect her daughter from school on the day of the incident, and another woman who rarely left her apartment. "It could have been me", she said.
Marseille city council has evacuated and rehoused 100 residents from neighbouring buildings and says heavy rain may have contributed to the collapse. However, according to neighbors, it is at the time of the tragedy could be people, particularly the homeless. A 2015 report by the government found that 100,000 residents of Marseille "were living in housing that was unsafe to their health or security", according to AFP.
Djaffar Nour, who was shopping down the street, said the collapse had happened in "a matter of seconds".More news: Trump hopes to meet N.Korea leader next year but counsels patience
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