More than 24 hours after the four-story building imploded, with residents reporting a strong smell of gas, dozens of firefighters are still battling a blaze that has hampered search operations.

However, the mayor of the Mediterranean port city said rescue workers had not given up hope of finding survivors.

"There is still hope, and as long as there is hope, we will not stop," said mayor Benoit Payan, at the scene of the disaster.

He said emergency workers were carrying out a "surgical" operation at the site, to "protect at all costs" any potential survivors.

The building was believed to have one apartment on each floor.

The fire at the site has made it hard for sniffer dogs to detect more victims or survivors.

"Given the particular difficulties of intervention, the extraction [of the bodies from the site] will take time," the fire department said in a brief statement announcing the bodies had been found.

On Sunday, before the discovery of the bodies, local prosecutor Dominique Laurens told reporters that eight people "were not responding to phone calls".

Five people from neighboring buildings sustained minor injuries in the blast and collapse, which occurred around 12:40 a.m. on Sunday.

The cause of the explosion is still to be determined, but investigators are looking at the possibility it was the result of a gas leak.

"I was sleeping and there was this huge blast that really shook the room. I was shocked awake as if I had been dreaming," said Saveria Mosnier, who lives on a street near the site in the La Plaine neighbourhood.

"We very quickly smelled a strong gas odor that hung around; we could still smell it this morning," she told AFP.

Deputy mayor Yannick Ohanessian told journalists at the scene several witnesses had reported "a suspicious smell of gas".

Evacuation

Two buildings next to the destroyed property were severely damaged, with one collapsing later in the day without injuring any rescuers.

Almost 200 residents have been evacuated and 50 have requested to be urgently rehoused.

An aid center for people looking for missing family members or loved ones has been opened in a neighboring district.

"A lot of families in the neighborhood are afraid," said Arnaud Dupleix, the president of a parents' association at the nearby Tivoli elementary school, which sprang into action to coordinate aid for those evacuated.

In 2018, eight people were killed in Marseille when two dilapidated buildings in the working-class district of Noailles caved in.

That disaster cast a harsh light on the city's housing standards, with aid groups saying 40,000 people were living in shoddy structures. 

But authorities on Sunday appeared to rule out structural issues in the latest collapse.

"There was no danger notice for this building, and it is not in a neighborhood identified as having substandard housing," said Christophe Mirmand, prefect of the Bouches-du-Rhone region.