More than 100 firefighters are working against a ticking clock to extinguish flames deep within debris to save up to 10 people possibly buried after a building exploded and collapsed in the French port city of Marseille.

Interior minister Gerald Darmanin said at least four people were known to live in the collapsed building and as many as 10 may have been there, though persistent flames and fears of further collapse prevented rescuers from being able to search for victims some 15 hours after the explosion.

“We cannot intervene in a very classic way,” Mr Darmanin said during a visit to the site, about 11 hours after the five-storey building collapsed shortly before 1am on Sunday.

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.

It was not known if anyone was killed, or what triggered the blast, Mr Darmanin said.

Firefighters, with the help of urban rescue experts, worked through the night and all day on 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.

Some 30 buildings in the area were evacuated, Mr Darmanin 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 in the centre of Marseille, adding to an array of difficulties for firefighters and rescue workers.

The intense heat made it impossible to send in dog teams to search.

Robots were reportedly being deployed.

A crane was brought in to clear rubble and firefighters were at one point seen in TV video hosing parts of the debris from a window in a nearby apartment as plumes of smoke rose skyward.

Marseille mayor Benoit Payan said two buildings that share walls with the one that collapsed were partially brought down before one later caved in.

It was among the evacuated structures.

Six people were taken to hospital.

A dog from the firefighters’ canine unit was seen sniffing debris, apparently at the neighbouring building that caved in.

“We’re trying to drown the fire while preserving the lives of eventual victims under the rubble,” Lionel Mathieu, commander of the Marseille fire brigade, said during a televised briefing.

“Firefighters are gauging minute by minute the best way to put out the fire,” Mr Payan said.

“We must prepare ourselves to have victims,” he said grimly.

An explosion was the “probable” cause of the building collapse, Mr Payan said, but later stressed that “no conclusions can be drawn” without an investigation.

The collapsed building is located in an old quarter in the centre of France’s second-largest city.

The noise from the explosion resounded in other neighbourhoods.

Nearby streets were blocked off.

French President Emmanuel Macron and Prime Minister Elisabeth Borne both tweeted their thoughts for people affected and their thanks to the firefighters.

In 2018, two buildings in the centre of Marseille collapsed, killing eight people.

Those buildings were poorly maintained – not the case with the building that collapsed on Sunday after an explosion, the interior minister said.