A major operation is under way to tackle the blaze, which broke out in the afternoon, and an area surrounding the building in central Paris has been cleared, it quoted officials as saying.
CNN reported that its spire had collapsed even as fire-fighters raced to tackle the blaze.
Paris Mayor Anne Hidalgo termed it a "terrible blaze" and urged people to cooperate with the fire-fighters while President Emmanuel Macron has cancelled his planned speech to the nation in light of the fire, which has engulfed the giant spire of the cathedral, an Elysee Palace official said.
Macron, in a tweet in French, said that he was "sad tonight to see this part of us burn."
US President Donald Trump, in a tweet, said "So horrible to watch the massive fire at Notre Dame Cathedral in Paris. Perhaps flying water tankers could be used to put it out. Must act quickly!"
Last year, the Catholic Church in France launched an urgent appeal for funds to save it after cracks began appearing in the structure of the popular tourist attraction.