PARIS (AFP) - Paris prosecutors have opened a preliminary investigation into the assets in France of Rifaat al-Assad, the uncle of the Syrian president, a judicial source said on Monday.

The probe follows a criminal complaint filed on September 13 by anti-corruption groups Sherpa and Transparency International, alleging the 66-year-old had illegally acquired "extraordinary wealth".

Once a stalwart of the Syrian regime, Rifaat al-Assad broke with the government of his brother, then-president Hafez al-Assad, in 1984 and settled in Europe.

Hafez al-Assad was the father of current Syrian President Bashar al-Assad, whose regime is fighting in a civil conflict that has left more than 110,000 dead since it began in March 2011.

Before splitting from the regime, Rifaat al-Assad was accused of being responsible for the deaths of thousands during the crushing of an Islamist uprising in 1982.

The criminal complaint accuses Rifaat al-Assad of acquiring wealth "in the billions of euros" through corruption, embezzlement of public funds, misuse of corporate assets and other crimes.

French media have reported that his holdings include a mansion and several dozen apartments in Paris.