Geologists in Morocco on Tuesday denounced the planned auction of a 66-million-year-old marine dinosaur skeleton in Paris and demanded that the remains be repatriated.

The nearly nine-metre-long plesiosaur, a marine reptile with a long neck and turtle-like flippers, was discovered in a phosphate basin near the Moroccan city of Khouribga.

The skeleton is set for auction on March 7 at the Drouot auction house in Paris.

"The plesiosaurs were among the most emblematic 'giant primitive beasts' which have intrigued scientists and amateurs for centuries," Drouot said in its auction catalogue, adding that the creatures had probably inspired the legend of the Loch Ness monster in Scotland.

But the Association for the Protection of the Geological Heritage of Morocco (APPGM) on Tuesday criticised the planned auction, saying the skeleton was a "unique patrimonial treasure".

It said it had asked Morocco's chief geologist to find out if it had been authorised for export.

If not, the association would ask the Moroccan authorities to "intervene with the organisers of the sale to restitute the plesiosaurus to its country of origin, Morocco", it said.

Alexandre Giquello, auctioneer and president of Drouot, said the creature was "reconstituted in Europe from four fossils legally bought at the Frankfurt Trade Fair" by Italian collectors.

A Moroccan company had offered them for sale along with the necessary export papers, he said.

"I understand that this sale creates a stir, but we should also respect the private initiative that allowed the reconstruction of this plesiosaur, which is part of humanity's heritage," he added.

Drouot said the specimen had been screened for customs compliance and "checked by customs and heritage authorities in Italy" prior to being prepared for sale.

The skeleton, which Drouot says is 75-percent complete, will be on display at the Paris auction house from March 4-7.

APPGM said it was likely to fetch around $477,000 (450,000 euros).

"This specimen is the only complete specimen of... museum quality on the market, and the first available for a private collection," Drouot said.

"The only existing comparable specimen is exhibited in the United States at the Wyoming Dinosaur Center in Thermopolis."

Moroccan media, citing an official at the ministry of energy and mines, said the remains had been smuggled out of the country. 

The Medias24 website said the foreign ministry had been asked "to inform the French authorities of the illegality of the sale and to request the return of the skeleton to its country of origin".

The Moroccan authorities have not commented publicly on the subject.

APPGM said Morocco's geological heritage was globally recognised for its richness and diversity.

But it added that "in the absence of regulatory texts... this heritage is undergoing a systematic and continuous deterioration."