Marseille: An apartment building collapsed in an apparent explosion Sunday in the French Mediterranean city of Marseille, injuring five people, with authorities warning there are likely to be fatalities as firefighters battled a blaze.

At the moment of the blast, around 12:40 am (2240 GMT), "everything shook, you could see people running and there was smoke everywhere, the building fell onto the street," local grocer Aziz told AFP, asking that his family name not be used.

"We have to be prepared to have fatalities in this terrible tragedy," Marseille mayor Benoit Payan told journalists at the scene in the central La Plaine district, where over 100 firefighters were still getting the blaze under control and the smell of smoke hung in the air.

Five people were injured in neighbouring buildings which were damaged by the collapse, and 33 were taken into care by emergency responders.

It is unclear how many people were inside the fallen block -- believed to have one apartment on each floor -- at the time of the collapse.

"Not all the people who were supposedly inside the building have been seen, families are worried," Housing Minister Olivier Klein told broadcaster Franceinfo.

The intense heat as the building burns has kept search dog teams from picking through the rubble.

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

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 their 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.

Interior Minister Gerald Darmanin was expected on the scene of the disaster on Sunday morning.

- Cause of blast unclear -

"We have to be very cautious about what the cause was at this stage," said Christophe Mirmand, the prefect of the southern Bouches-du-Rhone region. Gas was "one possible option," he added.

Gilles, who lives on a side street near the fallen building, told AFP the sound of the crash "was huge".

"It sounded like an explosion," said Gilles, declining to provide his last name.

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 Mirmand.

Further back in Marseille's history, eight people were killed in a 1981 building collapse, five in a 1985 explosion and four in a 1996 gas blast that demolished a seven-storey building.