Architects have said it could take up to 15 years to restore Notre Dame after French President Emmanuel Macron promised it would be done in five.
In an address to the nation days after the famous 800-year cathedral suffered severe damage from a fire, Mr Macron said the renovations to restore the iconic 19th-century spire, vaulting and two-thirds of its roof of the building would be completed in time for the Paris 2024 Olympics.
“We will rebuild the cathedral to be even more beautiful and I want it to be finished within five years,” he said.
French Prime Minister Edouard Philippe, who officially supported Mr Macron's timeframe, acknowledged that it would be difficult.
“This is obviously an immense challenge, a historic responsibility,” Mr Philippe said.
Experts have questioned if five years is enough time for such a massive operation.
Prominent French conservation architect Pierluigi Pericolo, who worked on the restoration of the 19th century Saint-Donatien basilica which was badly damaged by fire in 2015 in Nantes, told Inrocks magazine it could take triple that time.
“No less than 15 years … it’s a colossal task,” Mr Pericolo said.
He said the first stage of the operation would take “two to five years” and that would just be to check the stability of the massive cathedral that dominates the Paris skyline.
“It’s a fundamental step, and very complex because it’s difficult to send workers into a monument whose vaulted ceilings are swollen with water,” Mr Pericolo told France-Info.
“The end of the fire doesn’t mean the edifice is totally saved. The stone can deteriorate when it is exposed to high temperatures and change its mineral composition and fracture inside.”
Notre Dame’s rector said he would close cathedral to all tourists and services for “five to six years” acknowledging that “a segment” of the near 900-year-old edifice may be gravely weakened.
As more news of the disastrous fire emerges, people are taking heart in all of the objects that were saved from the inferno, especially the Cathedral's ancient rose windows.
Spokesman Gabriel Plus told reporters the rose windows were “in good condition” but that “there is a risk for the gables that are no longer supported by the frame”.
He praised firefighters who took down statues inside the gables above the rose windows to protect them and took care not to spray water too hard on the delicate stained glass.
In a sign of how big the operation to restore the building will be Mr Plus said firefighters and experts are still determining how much damage the structure suffered and what needs to be dismantled to avoid collapse.
A Paris fire official said the towers of Notre Dame would have fallen if firefighters had not deployed massive equipment and acted swiftly.
Philippe Demay denied there was any delay and said firefighters acted as fast as they could.
Mr Demay told reporters that the operation was extremely difficult and that the towers could have collapsed “if we hadn’t put heavy equipment in place”.
Almost a billion euros has poured in from ordinary worshippers and high-powered magnates around the world to help fund the restoration.
Presidential cultural heritage envoy Stephane Bern told broadcaster France-Info on Wednesday that 880 million euro (£762 million) has been raised in just a day-and-a-half since the fire.
Experts have been quick to estimate the restoration would cost into to the hundreds of millions and not billions, although it is too early to be certain.
Mr Philippe said a competition will be held to see if the spire should be rebuilt.
“The international competition will allow for the question to be asked, should the spire be rebuilt?” he said. “Should we rebuild the spire envisaged and built by Viollet-le-Duc under the same conditions … (or) give Notre Dame a new spire adapted to the technologies and the challenges of our times?”
Viollet-le-Duc was a famous French architect who restored many of Frances medieval buildings in the 1800s, including Notre Dame, that had suffered centuries of neglect.