Up to 10 people may be buried under the debris of a building that collapsed following an explosion in France’s port city of Marseille, but a fire deep within the rubble hindered rescue efforts Sunday, the French interior minister said.

Officials determined that at least four people lived in the building, Interior Minister Gerald Darmanin said.

It was not known if anyone was killed in the collapse or explosion, or what triggered the blast, he said.

Darmanin, who visited the site, provided the update nearly 11 hours after the five-story building collapsed before 1 a.m. More than 100 firefighters, aided by specialists, worked through the night to access and extinguish the fire, which the minister said was burning a few meters (feet) under the debris.

The delicate operation proceeded with the aim of keeping firefighters safe, preventing further harm to people potentially trapped in the rubble and not compromising vulnerable buildings nearby.

Some 30 buildings in the area were evacuated, Darmanin said.

The building that collapsed is located on a narrow street in the center 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.

Marseille Mayor Benoit Payan said that 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 hospitalized.

“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,” Payan, the mayor, said.

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

An explosion was the “probable” cause of the building collapse, 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 center of France’s second-largest city.

The noise from the explosion resounded in other neighborhoods.

Nearby streets were blocked off.

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