As many as 10 people were thought to be under the rubble after an explosion that caused two residential buildings to collapse in the southern French city of Marseille, authorities say.

The cause of the explosion was not yet known.

The collapse started a fire that complicated rescue efforts and that French Interior Minister Gerald Darmanin said could take hours to put out.

He said authorities estimated there were between four and 10 people under the rubble.

“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.

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

More than 100 firefighters were trying to put out fires under huge piles of rubble in a slow and delicate operation aimed at keeping firefighters safe and trying not to harm people potentially trapped.

Five people were taken to hospital with serious but not life threatening injuries and a sixth person was being treated for shock, he added.

A third building had partially collapsed and people had been evacuated from some 30 buildings in the area.

The buildings that collapsed on the Rue de Tivoli were not known to have any structural problems, Mr Darmanin said.

“Thoughts are with Marseille,” President Emmanuel Macron said in a Twitter message.

Mayor says explosion probably caused collapse

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.

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

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 was heard across multiple neighbourhoods.

Reuters/AP