French Interior Minister Christophe Castaner said air pockets under the debris meant there was hope of finding someone who could be saved.
The buildings - one condemned and allegedly vacant, the other containing apartments - gave way after 9am local time.
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.
Google Maps images taken in recent months showed the collapsed buildings had large visible cracks in their facades.
Castaner said inspectors had visited number 65 on October 18, and were concerned but did not see cause to evacuate.
Mr Tarabeux said: "It is very, very likely to be five people".
Images of the buildings before they collapsed, visible on Google Street View, showed that one of the buildings was clearly in poor fix, with boarded-up windows and large visible cracks on the facade before it collapsed. He has had no news of her since Monday. "It could have been me", she said.
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Sophie Dorbeaux, who lived in the now-demolished 65 rue d'Aubagne, meanwhile told AFP she had left on Sunday night to stay with her parents because "for days, the doors of some of the apartments wouldn't close".
The neighbourhood is home to many buildings in a similarly poor condition, some of them run by slum landlords.
Residents say they had warned the buildings were structurally unsound for years, but accused city authorities of doing little.
The buildings were in a small shopping street in the centre of the city.
Leading left-wing politician Jean-Luc Melonchon, who visited the scene Monday, had reportedly been critical of the city's housing policy. In 2011 the local authorities began a plan to renovate the city centre, but a 2015 government report suggested that 100,000 Marseille residents were living in housing unsafe to their health or security.
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