They have so far recovered the remains of six people known to have been in the four-storey apartment block when it was destroyed in the early hours of Sunday.

The explosion shook the whole Camas neighbourhood, a few hundred metres from Marseille’s historic old port.

“The toll is unchanged and operations are continuing,” a fire service spokesman told AFP on the scene early on Tuesday.

Five women and three men, most aged between 66 and 89 but including a couple aged 29 and 31, are known to have been in the building when it fell.

“It would be a miracle to find any survivors but we have faith,” said a priest, Father Olivier, at a Monday prayer vigil in the nearby  Saint Michel church.

Police forensics experts are working to identify the bodies retrieved so far.

But eyes are now turning to possible causes for the overnight blast, with many witnesses recalling smelling gas around the time of the explosion.

As well as 22 forensics officers, 18 detectives are on the scene sifting for evidence. Authorities are yet to give any preferred theory of what happened.

Meanwhile, around 200 people evacuated from the neighbourhood face an uncertain wait before they are allowed to return to  their homes.

One building adjoining the fallen block largely collapsed a few hours later, while the structure on the other side was weakened and risks falling in turn.

Other houses on the street may have suffered less visible damage, meaning they have to be torn down, Marseille’s deputy mayor for security, Yannick Ohanessian, said on Monday.

Some residents were allowed to return briefly on Tuesday to recover vital items from their homes, given just a few minutes to choose between important papers, clothes, medicines or a bicycle for the daily commute.

“The worst thing is not knowing how long it’s going to be. I’m most worried not to know where I’ll be living, whether I’ll need to find a new apartment,” said Alhil Villalba, 33.