France's top administrative court on Wednesday confirmed the conviction of Rifaat al-Assad, uncle of Syrian leader Bashar al-Assad, in an "ill-gotten gains" case over wealth estimated at 90 million euros ($89 million).

Rifaat al-Assad, 85, is the younger brother of Bashar's father and former Syrian dictator Hafez al-Assad, and himself held the office of vice president but fled the country in 1984 after a failed coup.

He had made a final appeal to France's Court of Cassation after a lower court last year confirmed his four-year jail sentence for conspiracy to launder Syrian public funds between 1996 and 2016.

In the same judgement, he was convicted of concealing serious tax fraud and employing servants off the books, with authorities confiscating a slew of his properties.

Rifaat al-Assad, who has not attended hearings due to ill health, insists that his property empire stretching across Spain, France and Britain stems from gifts from Saudi crown prince and later king Abdullah, who died in 2015.

The case is the second in France under a law passed last year targeting fortunes fraudulently amassed by foreign leaders.

Teodorin Obiang, the eldest son of the president of Equatorial Guinea, was convicted last year to a three-year suspended sentence and 30 million euros in fines.

- War crimes case -

In Syria, Rifaat al-Assad was the head of the elite Defence Brigades, internal security forces that violently quashed a 1982 Islamist uprising in the city of Hama.

Having stayed away for three decades following his failed attempt to seize power, pro-government media reported that he returned to Syria last autumn.

In 1984, he had fled first to Switzerland then France, where he was given the Legion of Honour -- the country's top award -- in 1986 for "services rendered".

French investigators opened a probe into his property holdings in 2014 after complaints from watchdogs Transparency International and Sherpa.

They seized two Paris townhouses, dozens of apartments in chic neighbourhoods of the French capital and office spaces.

Since then, around 80 of his former employees living at an estate outside Paris have been mostly without water and electricity as no one was paying the bills.

While Rifaat al-Assad's age and poor health mean he is unlikely ever to serve jail time in France, Wednesday's ruling confirms the confiscation of the properties for good.

That could set up Syria as one of the first countries to potentially benefit from a scheme to return funds recovered under the ill-gotten gains law.

Rifaat al-Assad also faces a court case in Spain over far larger suspicions of ill-gotten gains covering 500 properties, as well as a prosecution in Switzerland over war crimes dating back to the 1980s.