French prosecutors have launched an arson investigation following a major fire at a 15th-century Gothic cathedral in the western city of Nantes on Saturday.

The blaze spewed smoke from the front windows of the Cathedral of Saint Peter and Saint Paul, but the damage was mostly limited to the large pipe organ inside.

The organ was “completely destroyed”, said the head of the local fire department, Laurent Ferlay.

The public prosecutor’s office is now investigating whether arson was the cause and experts from Paris are expected to provide support.

The fire broke out in three places, which is probably not a coincidence, Nantes prosecutor, Pierre Sennes, told local radio station France Bleu Loire Ocean.

However, no signs of a break-in have been found so far, Sennes said, according to news agency AFP.

Flames originated from the organ and on both sides of the nave, BFMTV reported, citing the public prosecutor’s office.

According to media reports, over 100 firefighters were deployed to the cathedral to extinguish the blaze, which broke out around 7.30 a.m. (0530 GMT).

By Saturday afternoon the fire was nearly completely out but work is expected to continue into the night.

French media reported that a crane was to lift the wreckage of the organ out of the church in order to reach the last burning embers.

The diocese published pictures of the damage that showed debris from the organ scattered in the church’s interior.

The main organ is completely gone, diocese administrator, Francois Renaud, told AFP after visiting the cathedral.

He said no price could be placed on the loss.

The organ was built in 1619 and has been rebuilt and restored several times since.

In a video posted by firefighters shortly after the blaze erupted, flames could be seen through the church’s front windows while smoke pours out of them.

The windows were also destroyed.

According to the Nantes diocese, the church had also caught fire in 1972, destroying its roof.

After restoration work, the church, which belongs to the French late Gothic style, reopened its doors in 1985.

In 2015, a spectacular fire destroyed the roof of another church in Nantes, the Basilica of Saint Donatian and Saint Rogatian.

Saturday’s fire comes a little more than a year after Notre Dame Cathedral in the capital, Paris, caught fire, destroying the roof of the 12th-century Gothic cathedral as well as its 93-metre-high spire.

French President Emmanuel Macron expressed his support for firefighters saving this “Gothic jewel”.

After reaching Nantes, French Prime Minister Jean Castex likewise thanked local firefighters for their work.

He assessed the damage together with Interior Minister, Gerald Darmanin, and Culture Minister, Roselyne Bachelot.

Castex said reconstruction should start as soon as possible.

The state will play its part, he promised. (DPA)