The search has proven hard because one of the collapsed buildings, 63 Rue d'Aubagne, was closed and was possibly used by squatters.
City officials have evacuated more than 100 nearby buildings following the collapse, which authorities believe may have been caused by heavy rains, AFP reports.
As many as eight people are feared dead in total after buildings collapsed suddenly on Monday morning in Noailles, a working-class district in the heart of the French port city of Marseille.
"We've found some pockets of air that means we still have some hope of finding and identifying a survivor", Castaner said.More news: Bargains and special deals available on Amazon for Black Friday sale 2018
Marseille firefighters say the death toll has risen to six in the collapse of two dilapidated apartment buildings in the southern French city with the discoveries of two men's bodies Wednesday.
"People died for nothing, even though we knew".
Google Maps images taken in recent months showed the two collapsed buildings had large visible cracks in their facades.
A local resident, Toufik Ben Rhouma, blamed Marseille city hall for the disaster.
He added he had seen two other people arriving to visit one of the tenants in the building, which had 12 apartments, nine of them occupied.More news: 2-month-old Boeing plane crashed in Indonesia after key sensor replaced
A young bar waiter, with tears in his eyes, watched the scene and anxious about an Italian woman who lived in the building.
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.
"I haven't had any news", he said, wandering among the rescuers.
Marseille authorities began a vast upgrade plan for the city centre in 2011.
According 2015, about 100 thousand inhabitants of Marseille, live in housing that threatens their health or safety.More news: Eurogroup supports Commission, urges Italy to submit new budget plan
Djaffar Nour, who was shopping down the street, said the collapse had happened in "a matter of seconds".