The uncle of Syrian President Bashar al-Assad is under investigation in France for amassing a 90 million euro ($A125 million) fortune - including a stud farm and luxury apartments - despite being kicked out of Syria "with nothing" 30 years ago.

French investigators have for a year been probing the finances of Rifaat al-Assad, the younger brother of late dictator Hafez al-Assad.

Rifaat has spent more than 30 luxurious years moving between homes in Paris, London and the southern Spanish city of Marbella since he was forced into exile for trying to seize power from his brother.

His family's assets, outlined by French customs in a May 2014 report, are valued at around 90 million euros, much of it held through a web of businesses based in Luxembourg.

The inventory includes a stud farm near Paris, as well as two mansions, two apartment blocks and a plot of land in the French capital.

Rifaat told investigators he "had nothing" when he left Syria, having always given his wages away to the poor, according to a source close to the investigation.

The investigation into Rifaat's finances was triggered by Sherpa, an activist group representing the victims of financial crime, which claims the fortune was stolen during his time at the heart of the Syrian regime.

The family claims it is the result of gifts from wealthy Saudi supporters, including former king Abdullah, with whom Rifaat shared a love of horse-racing.