Residents are believed to be buried under the rubble, as firefighters work to extinguish flames following an explosion.

Eight people are not responding to calls and are thought to be under the rubble of two buildings that have collapsed in an explosion near the port in the southern French city of Marseille, local officials have said.

More than 100 firefighters worked against a ticking clock to extinguish flames deep within the rubble of the five-storey building, but more than 17 hours later “the situation is not yet stabilised,” Marseille Prosecutor Dominique Laurens said at an evening news conference on Sunday.

The collapse caused a fire that has complicated rescue efforts and investigations, and it has not been brought under control, she told a news conference.

Laurens said police have yet to confirm the apparent disappearance of a ninth person who lived in a next-door building. Five people suffered minor injuries from the collapse, which occurred shortly before 1am (22:00 GMT).

Laurens said the cause of the explosion was not yet known.

Television footage showed clouds of smoke rising from the rubble as firefighters tried to put out the fire, while trained dogs were used to try to locate victims.

“We have nothing, not even an ID card. We have lost everything,” said a man who gave his name only as Roland, in an interview with local newspaper La Provence.

He managed to get out of the building on 15 Rue de Tivoli with his wife and two children before it collapsed, together with a neighbouring building. A third building partly collapsed.

Interior Minister Gerald Darmanin, who visited the site, said 30 buildings in the area were evacuated.

Marseille Mayor Benoit Payan said two buildings that share walls with the one that collapsed were partially brought down before one later caved in, another complication in the search-and-rescue operation. The buildings were among the evacuated structures.

Drones and probes have been used to examine the scene for signs of life. The burning debris was too hot for dogs in the firefighters’ canine team to work until Sunday afternoon, though smoke still bothered them, the prosecutor said.

“We cannot intervene in a very classic way,” Darmanin said during a morning visit to the site. He said the fire was burning a few metres under the mounds of debris and that both water and foam represent a danger to victims’ survival.

An investigation has been opened for involuntary injury, at least initially sidestepping possible criminal intentions. A gas explosion was among the tracks to check, said Laurens, the prosecutor. But the start of the probe also was limited by the heat of the blaze.

Firefighters, with the help of urban rescue experts, worked through the night and all day Sunday in a slow race against time. The delicate operation aimed to keep firefighters safe, prevent further harm to people potentially trapped in the rubble and not compromise vulnerable buildings nearby, already partially collapsed.

Laurens, the prosecutor, said that firefighters “are really in a complicated situation, dangerous for them”. Work is progressing but with safety precautions, she said.

“We heard an explosion … a very strong explosion which made us jump, and that’s it,” said Marie Ciret, who was among those evacuated. “We looked outside the window at what was happening. We saw smoke, stones, and people running.”

The building that collapsed is located on a narrow street less than a kilometre (a half-mile) from Marseille’s iconic old port, adding to an array of difficulties for firefighters and rescue workers.