The French general appointed to oversee the reconstruction of Notre Dame Cathedral has said the iconic building is still at risk of collapse more than six months into the efforts to restore it.

Gen Jean-Louis Georgelin said the cathedral is “not saved yet” and has to undergo a delicate operation to remove fused scaffolding around the spire, destroyed by a devastating fire last April.

“The cathedral is still in a state of peril,” Georgelin told the French broadcaster CNews. “There is an extremely important step ahead, which is to remove the scaffolding,” he said.

Georgelin, who has described the conservation and restoration of the cathedral as “an immense challenge”, added that the condition of the ceiling vaults was not fully known and he could not guarantee they “won’t fall apart”. However, he said initial observations on the current state of the 12th-century edifice were “reassuring”, adding: “We feel quite confident.”

Facebook Twitter Pinterest Partially destroyed scaffolding that was in place before the fire. Photograph: Stéphane de Sakutin/AFP/Getty Images

The cathedral’s rector, Monseigneur Patrick Chauvet, said a fortnight ago the cathedral was so fragile there was a “50% chance” the structure may not be saved if the scaffolding fell on stone vaults weakened by the inferno.

“It will be out of danger when we take out the remaining scaffolding,” Chauvet said.

“Today we can say there is maybe a 50% chance that it will be saved. There is also a 50% chance of the scaffolding falling on the three vaults, so as you can see the building is still very fragile.”

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In November, the archbishop of Paris, Monseigneur Michel Aupetit, said work on the cathedral was still in the “diagnostic” phase and final evaluation of the damage should be finished this spring.

Aupetit said the operation to remove 500 tons of scaffolding, which came down when the spire collapsed, was a delicate one.

“We will have to encircle the scaffolding, then put a second scaffolding over it. From this new scaffolding, workers will descend by rope and cut it bit by bit into small pieces and this will take a long time,” he said.

Afterwards, the vault would have to be examined stone by stone, he added.

“We cannot take any risks … we have to know which ones need replacing and which ones to keep … only then will we know how much [the repairs] will cost and how long they will take.”

Play Video 1:11 Notre Dame Cathedral: before and after the devastating fire – video

For the first time since the French Revolution there were no Christmas services in the cathedral this year.

Georgelin was appointed to oversee the restoration of Notre Dame by the French president, Emmanuel Macron, who in the wake of the fire said the cathedral would be rebuilt within five years. Experts have thrown doubt on whether the work can be completed in that time. Georgelin denied reports that the wooden roof structure – known as “la forêt” (“the forest”) because of the quantity of wood involved – would be rebuilt exactly the same, from wood.

“There will be a study and all possible options will be looked at,” Georgelin said. Once the extent of the damage is known, work is expected to begin in 2021.