Rebuilding the Notre-Dame Cathedral in Paris could take decades after it was gutted by a fire, experts warned Tuesday, even as its top priest expressed hope he could celebrate mass there within years.

Parisians and people around the world watched in horror on Monday as flames ripped through the roof of the beloved 850-year-old Gothic cathedral, causing the spire and most of the vaulted roof to collapse.

"We will rebuild Notre-Dame together," French President Emmanuel Macron vowed after assessing the damage, declaring that the disfigured cathedral had been spared "the worst".

France has experience of reconstructing cathedrals, including one in Reims that was severely damaged by shelling during World War I and another in Nantes that was gutted by fire in 1972.

Asked how long the rebuild could last, Eric Fischer, head of the foundation in charge of restoring the 1,000-year-old Strasbourg cathedral, which recently underwent a three-year facelift, said: "I'd say decades."

"The damage will be significant. But we are lucky in France to still have a network of excellent heritage restoration companies, whether small-time artisans or bigger groups," he told AFP.

Fischer said the ability to rebuild the colossal cathedral in a manner that respects its original form and character would depend on the plans, diagrams and other materials available to the architects.

They would need "a maximum of historical data or more recent data gathered with modern technology such as 3D scans" of the kind used in the restoration of the Strasbourg cathedral, he said.

'Not in my lifetime'

The French government's representative for heritage, Stephane Bern, said that money would not be the problem.

Within hours, pledges of donations amounting to nearly €700 million had flooded in from some of France's richest families and companies and foreign governments were lining up with offers of help.

Bern, a 55-year-old TV presenter famous for his programmes on medieval France, said he feared it would not reopen in his lifetime.

"It will be rebuilt for future generations," he said.

A symbol of Paris for close to a millenium, serving as a sanctuary for the hero in Victor Hugo's classic novel "The Hunchback of Notre-Dame", the towering house of worship has been in the wars before.

During the French Revolution its treasures were plundered and the figures of kings carved into the stone above its entrance doors defaced.

Deemed unstable the spire was dismantled in 1792 and the cathedral fell into a state of disrepair until the mid-19th century when architect Eugene Viollet-le-Duc gave the famed structure a major makeover.

But the intricate wooden oak frame that held up the roof, the so-called "forest", had stood the test of time since its construction in 1220-1240 - until being consumed by Monday's inferno.

For carpenters, "it's a bit as if the Mona Lisa went up in smoke," Thomas Buechi, head of Charpente Concept which specialises in timber frames, told AFP.

Recreating it will be the trickiest part of the restoration, experts said.

France's top producer of oak said he was worried the country did not have enough of the precious timber for the job.

Sylvain Charlois estimated that around 1,300 oak trees had been used in the construction of the original roof.

"To constitute a big enough stock of oak logs of that quality will take several years," he said.

Tighter deadline needed?

Francois Jeanneau, one of the 40 architects in charge of state monuments, suggested that Paris draw on the example of Nantes cathedral and build a new "forest" of reinforced concrete.

"The un-initiated can barely tell the difference," he told Le Parisien newspaper.

Despite the longer forecasts of decades of work, the rector of Notre-Dame, Monsignor Patrick Chauvet, said he was hopeful of being back behind the pulpit before he retired.

"I'm 67 now and if all goes well, even if it takes 10 years, I will be 77 and still able to do it," he told France Inter radio.

Jack Lang, who served as a hugely prominent culture minister under late president Francois Mitterrand, called talk of a decade-long restoration programme "a joke".

"We have to give ourselves a tighter deadline, like we have done in the past on major projects."