French police were on Thursday probing theft accusations linked to the sale of valuable stained glass that once adorned Notre Dame, with auction house Sotheby's insisting it had done everything by the book.

Two stained glass pieces that disappeared from the landmark cathedral in Paris in 1862 went on the block at Sotheby's more than a century and a half later, in 2015.

One of the stained glass pieces represents an angel holding a candle, and the other an angel holding a censer. Their diameter is about 40 centimeters (16 inches).

The Sotheby's house sold them in 2015, one for 123,000 euros and the other for 111,000 euros ($132,000 and $119,000 at current rates).

A French association, "Lumiere sur le Patrimoine" ("Light on Heritage"), specializing in investigating cases of possibly stolen goods at public sales, alleges that the pieces had been robbed from Notre-Dame, and on Wednesday filed a legal complaint for theft, and for the handling of stolen goods.

French prosecutors told AFP they had launched a police investigation "for an initial analysis" of the allegations.

The pieces, believed to date back to the 13th century, were created as a pair, and part of the cathedral's main rose window on the northern side of its transept.

Sotheby's said at the time of the sale that they were believed to have been taken down by Eugene Viollet-le-Duc, a famous architect in charge of the cathedral's restoration, in 1862 and first sold by a stained glass restorer called Edouard Didron sometime between 1877 and 1905.

On Thursday, Sotheby's said it had respected the law and regulations at the 2015 auction.

"Before putting an item up for sale we proceed to all the research, diligence, and controls necessary to ensure that there is no legal obstacle to the sale," it said in an email to AFP.

The auction house said it had obtained all official authorizations, including export licenses, and notified experts and museums.

Sotheby's said it had not been contacted by the association filing the complaint.

Similar pieces were currently in the possession of the Art and History Museum in Geneva, it said.

Notre Dame, one of the French capital's most famous monuments, is currently being restored after its roof went up in flames in 2019.

Its spire, which toppled in the blaze, will rise again before the 2024 Summer Olympics in Paris, the new chief of the mammoth reconstruction project, Philippe Jost, said Thursday.

Towering 100 meters above ground level, the wooden spire will already be visible from the end of this year, gradually emerging from scaffolding as its roofing is attached, he said.