Eight people were unaccounted for Sunday after an apartment building collapsed in the French Mediterranean city of Marseille as a fire hindered rescue operations, authorities said.

Five people from neighbouring buildings were hurt in a possible explosion that destroyed the four-storey block at around 12:40 am (2240 GMT Saturday).

Local prosecutor Dominique Laurens told reporters that eight people "were not responding to phone calls", saying it was impossible so far to identify the cause of the collapse.

A gas explosion was "clearly" one of the possible causes, she added.

Multiple witnesses told AFP they had heard the sound of a blast.

"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 in a street near the site in the La Plaine neighbourhood.

"We very quickly smelled a strong gas odour that hung around, we could still smell it this morning," she added.

Smoke was still rising from the mound of rubble Sunday afternoon, an AFP photographer saw from a nearby building.

"We have to be prepared to have fatalities in this terrible tragedy," Marseille mayor Benoit Payan had earlier told journalists at the scene.

More than 100 firefighters were battling the blaze in the ruins of the building, believed to have one apartment on each floor.

The intense heat as the building burns has kept search dogs from picking through the rubble, with Interior Minister Gerald Darmanin saying it would be "several hours... maybe even longer" before the fire was out.

"Time is of the essence" to discover possible survivors among the ruins, Marseille fire chief Lionel Mathieu said.

The rescuers' task has been complicated by the partial collapse of one of the adjoining buildings, where eight people had to be brought down by ladder after taking refuge on a roof terrace.

Other buildings on the street were evacuated and around 180 residents put up in schools, while an aid centre for people looking for missing family members or loved ones has been opened in a neighbouring district.

 

- Cause of blast unclear -
 

Deputy mayor Yannick Ohanessian told journalists at the scene that "several witnesses have reached us this morning to say there was a suspicious smell of gas".

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

Eight were killed in Marseille in 2018 when two dilapidated buildings in the working-class district of Noailles caved in.

The accident cast a harsh light on the city's housing standards, with aid groups saying 40,000 people live in shoddy structures.

But authorities appeared to rule out structural issues in the latest collapse, in a neighbourhood known for its bars and nightlife.

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