Firefighters are 'not sure' that Notre-Dame Cathedral can be saved following a devastating fire which has destroyed much of the building.
The inferno continues to rage inside the building after the giant 100m spire earlier collapsed into the roof.
Flames have now spread into one of Notre Dame's iconic towers.
Paris fire chief Jean-Claude Gallet said it is unclear if city firefighters will be able to keep the blaze from spreading further and causing more destruction.
Speaking outside the cathedral, he said "we are not sure we are capable of stopping the spreading" to Notre Dame's second tower and belfry.
He said: "If it collapses, you can imagine how important the damage will be."
French deputy interior minister Laurent Nunez also told reporters it is ‘not certain’ that the cathedral can be saved, according to news agency AFP.
Around 400 firefighters are tackling the blaze.
Photos and videos on social media show the roof of the 850-year-old Gothic building covered in flames, after fire broke out earlier on Monday evening.
French President Emmanuel Macron postponed a televised speech to the nation because of the stunning blaze and was going to the cathedral himself.
He tweeted: "Our Lady of Paris in flames. Emotion of a whole nation. Thought for all Catholics and for all French. Like all our countrymen, I'm sad tonight to see this part of us burn."
The deputy mayor of Paris, Emmanuel Gregoire, said the cathedral had suffered "colossal damages", and the emergency services were trying to salvage the art and other priceless pieces stored in the cathedral.
A cathedral spokesman said the entire wooden interior was burning and likely to be destroyed.
Officials said the fire may be linked to renovation work at one of the world's most famous tourist attractions, French media reported.
Paris prosecutors opened an investigation into the blaze as the fire was still burning out of control, and the authorities said no-one had been hurt.
The Vatican has issued a statement that said: "The Holy See has seen with shock and sadness the news of the terrible fire that has devastated the Cathedral of Notre Dame, symbol of Christianity in France and in the world."
The statement said the Vatican is praying for firefighters "and those who are doing everything possible to confront this dramatic situation".
It also expressed "our closeness to French Catholics and the population of Paris, and we assure our prayers for firefighters".
One of the city's oldest and most recognisable buildings, work began on Notre Dame de Paris Cathedral in 1163.
The original structure was completed nearly 200 years later, in 1345, and its name literally translates to "Our Lady of Paris".
Some 13 million people now visit the Catholic landmark every year - more than 30,000 every day on average - according to its official website, and it is believed to be the most visited structure in the French capital.
