French President Emmanuel Macron vowed Tuesday that renovations to restore Notre Dame’s iconic 19th-century spire, vaulting and two-thirds of the cathedral's roof would be completed in time for the 2024 Paris Olympics.

However, some experts fear that the ambitious timeline of five years set by Macron is just too fast and unrealistic.

Even French Prime Minister Edouard Philippe — while supporting the government timeline — acknowledged in an address Wednesday that it would be difficult.

"This is obviously an immense challenge, a historic responsibility," Philippe said.

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Medieval churches took decades, even hundreds of years, to construct – Notre Dame itself took nearly 200 years to become what it was before Monday’s destructive fire.

Even with new technology afforded to construction and restoration crews today, experts predict that restoring the jewel of Gothic architecture would likely take much longer.

Pierluigi Pericolo, in charge of restoration and security at the St. Donatian basilica in Nantes, said it could take two to five years just to secure Notre Dame, given its size.

"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," he said on 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."

Emily Guerry, a professor of medieval history at the University of Kent in England, told NBC News that a restoration project for the 850-year medieval cathedral will take around two decades to complete – and even then, it wouldn’t be the same.

“This will be the largest, most important cultural renovation project in France for some time to come,” she said.

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The most critical part of the reconstruction would be replacing – or restoring – the wood structure that held much of the interior of the cathedral together and well as part of the roof destroyed in the blaze.

Known as the “Forest,” the wood roof is made up of centuries-old oak trees that were added to the cathedral in 1220. According to reports, when workers began constructing the roof, they cleared 50 acres of oak trees of threes that were already 300 to 400 years old at the time. That puts the oldest timber in the cathedral at nearly 1,300 years old.

Experts said the roof cannot be rebuilt exactly as it was because, at the moment, France does not have the trees of the size that they were cut in the 13th century.

"In the Middle Ages ... it was possible to find huge amounts of beautiful strong oak," Guerry told CBS News, but overuse led to the destruction of many of Europe's oak forests. "The ability to find around 3,000 more big, strong trees in the next two decades is going to be tricky."

This leads many to believe that Macron might be focusing on rallying a mourning country, instead of what is the reality.

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Peter Fuessenich, who oversees all construction work for the Gothic cathedral in Cologne, Germany, told broadcaster RTL on Tuesday that it could take decades to repair the damage.

“It will certainly take years, perhaps even decades, until the last damage caused by this terrible fire will be completely repaired,” he said, adding that it was “a tragedy with a European dimension.”

Notre Dame’s rector said Wednesday that he will close the cathedral for up to six years.

Bishop Patrick Chauvet acknowledged that the famed monument would close down for "five to six years" as he spoke with local business owners Wednesday.

He acknowledged that "a segment of the cathedral has been very weakened” but did not elaborate which section he was talking about.

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Nearly $1 billion in donations have poured in for the vast restoration of the fire-ravaged cathedral.

Experts have put this in the threshold of realism — estimating the restoration would cost into to the hundreds of millions, although they acknowledge it is too early to be certain.

Some criticism has already surfaced among those in France who say the money could be better spent elsewhere, on smaller struggling churches or workers.

Construction operations around the church were already underway as teams brought in a huge crane and delivered planks of wood to the site Wednesday morning. Firefighters were still examining the damage and shoring up the structure after Monday night's fire.

Macron is holding a special Cabinet meeting Wednesday dedicated to the Notre Dame disaster, which investigators believe was an accident possibly linked to renovation work.

But Paris prosecutor's office revealed that investigators have still not been able to look inside the cathedral, as it remains perilous.

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Some 30 people have already been questioned in the investigation, which the Paris prosecutor warned would be "long and complex." Among those questioned are workers at the five construction companies involved in work renovating the church spire and roof that had been underway when the fire broke out.

Meanwhile, it also emerged that it took cathedral staff 23 minutes to find the fire. An alarm first went off at 6:20 p.m. local time, but it wasn't until a second alarm went off at 6:43 p.m. that it was detected in the attic, according to Paris prosecutor Remy Heitz.

The cathedral – immortalized in Victor Hugo’s 1831 novel “The Hunchback of Notre Dame” – was undergoing a $6.8 million renovation project when the blaze broke out.

The 12th-century church is home to relics, stained glass and other works of art of incalculable value, and is a leading tourist attraction. Its organ dates to the 1730s and was constructed by Francois Thierry.

The Associated Press contributed to this report.