Rescue personnel work at the scene where a building collapsed in Marseille, France, on Sunday. AFP

More than six hours after the collapse of the five-story building before 1am, it was not known if anyone was killed.

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"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 blazes under huge piles of rubble in a slow and delicate operation aimed at keeping firefighters safe and trying not to harm people potentially trapped and not.

Rescue vehicles gather at the scene where a building collapsed in the southern French port city of Marseille. AFP

"Firefighters are gauging minute by minute the best way to put out the fire,” Marseille Mayor Benoit Payan said at a televised briefing.

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

Payan said two neighboring buildings were partially brought down and one was in danger of collapsing, creating a double challenge. Residents were evacuated and six were hospitalized, the mayor said.

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.

The mayor said that an explosion was the "probable” cause of the building collapse, but stressed that an investigation would ultimately determine the cause.

French Interior Minister Gerald Darmanin was expected at the scene later Sunday.

Associated Press