In the hours after the blaze, Macron promised a distraught French nation that the cathedral, which dates back to the 12th century, would be rebuilt and later said it would be reopened in some form to worshippers by 2024.

More than 700 days after workers scrambled to shore up Notre-Dame's flying buttresses, stabilise the bell towers and install hundreds of movement sensors, the effort to make the site secure ahead of restoration is almost complete.

From the rooftop, now largely covered by a complex scaffolding structure, work platforms and in places a tarpaulin "umbrella", Macron peered down into the cathedral's damaged transept and thanked workers on the site.

"We're all impressed with what we see, with the work that has been achieved in two years," Macron told a group of workers with the Paris skyline behind him. "Bravo and thank you."

"We're arriving at a critical juncture," General Jean-Louis Georgelin, the former army chief of staff named by Macron to head the renovation, told France Inter radio. Restoration work would begin before the end of 2021, he said.

The first phase of the project was complicated by the need to remove 200 tonnes of twisted metal after the fire consumed 40,000 pieces of scaffolding that had been erected around the spire at the time of the blaze.

Work was also halted during the first summer after the fire because of worries over lead contamination and has since at times been slowed by the coronavirus pandemic.

Financing for the restoration was not a concern yet, Georgelin said. Some 834 million euros in donations flooded in from billionaire tycoons and Catholic households in the aftermath of the fire.

"As things stand, we will need all these donations to complete the necessary work. We need people to keep giving money because it is a work without end," Georgelin said.

The cathedral, which featured in Victor Hugo's classic novel "The Hunchback of Notre-Dame", is a UNESCO World Heritage site into which 13 million visitors used to pour every year.

Australian Associated Press