A building has collapsed in the early hours of Sunday after a loud explosion ripped through the structure.
More than 100 firefighters rushed to the five-storey residential building in the port city of Marseille in southern France at around 12.40am (11.40pm BST).
A fire deep within the rubble is hindering rescue efforts, officials said, with it not yet known if anyone has been injured.
The noise of the explosion could be heard in nearby neighbourhoods, while two neighbouring buildings were partially brought down as a result of the blast.
Residents of neighbouring buildings were evacuated and six were taken to hospital, Marseille Mayor Benoit Payan said. He added that one further building was in danger of collapsing.
"Firefighters are gauging minute by minute the best way to put out the fire," Mayor Payan said in a televised briefing. "The lives of firefighters are also at play," he added.
Firefighters are trying to put out blazes under huge piles of rubble in a slow and delicate operation aimed at trying not to harm people potentially trapped.
Nearby streets have been blocked off while emergency services work at the scene, which is located in an old quarter in the centre of France's second-largest city.
Marseille fire brigade commander Lionel Mathieu said: "We're trying to drown the fire while preserving the lives of eventual victims under the rubble."
He added that the intense heat made it impossible to send in dog teams to search.
Pictures taken at the scene show large amounts of rubble piled up in the street. A number of fire firefighters can be seen standing around the collapsed building, with some even standing on top of rubble as the rescue operation continues.
Large plumes of smoke can be seen coming from one of the buildings, with Interior Minister Gerald Darmanin expected at the scene later on Sunday.
Mayor Payan said an explosion was the "probable" cause of the building collapse, but stressed that an investigation will ultimately determine the cause.
"We must prepare to have victims," he said.
