An arson investigation has been launched into a fire that broke out in a cathedral in the western French city of Nantes on Saturday, the city's Mayor Johanna Rolland told CNN affiliate BFM.

"What is certain is that there were three different fire outbreaks, I saw them myself," Rolland said about the blaze at Cathedral of Saint Peter and Saint Paul, which has been contained.

Fire Chief Laurent Ferlay was quoted as saying by the news channel that the blaze was not as big as the one that engulfed Notre Dame Cathedral in April 2019, and the damage is not as bad.

"The damage is concentrated on the organ, which seems to be completely destroyed. Its platform is very unstable and could collapse," he said.

At least 104 firefighters went to the spot to bring the flames under control.

Reacting to the fire mishap, President Emmanuel Macron tweeted: "After Notre-Dame, the Saint-Pierre-et-Saint-Paul cathedral, in the heart of Nantes, is in flames. Support for our firefighters who take all the risks to save this Gothic jewel of the city of the Dukes."