French prosecutors said Friday they had seized a house in the Paris suburbs linked to the son of the president of the Republic of the Congo, in a probe into suspected "ill-gotten gains".

The property in upscale Neuilly-sur-Seine, linked to Denis Christel Sassou Nguesso, "was seized in June", the office of the National Financial Prosecutor (PNF) confirmed to AFP after a report by investigative news site Mediapart.

However, they added that "Denis Christel Sassou Nguesso has so far not been charged".

Citing police documents, Mediapart reported that the townhouse was bought in 2009 for 5.2 million euros ($5.2 million) before being renovated for a further 5.4 million, and was "definitely" home to Denis Christel Sassou Nguesso and his family.

One of the owners of the property holding company that owns the building is the Congolese minister's chief of protocol, known to be one of Sassou Nguesso's "strawmen", the site added, citing an investigation by the OCRGDF serious financial crimes unit.

"I'm outraged that France, with its history as a colonial and slave-holding great power, is now coming to lay blame at African leaders' feet," Sassou Nguesso's lawyer Jean-Jacques Neuer said.

"Many very ill-gotten gains that belong to Africans are in French hands," he added.

Neuer insisted that the investigation of his client was "political and not judicial".

A 2007 complaint by watchdogs prompted Paris anti-corruption investigators to look more closely into the dealings of Congolese President Denis Sassou Nguesso, a former paratrooper who first came to power in the central African nation in 1979.

He and his family, as well as the Bongo family in power in Gabon, are believed to have misused public funds to acquire property empires in France.

Sassou Nguesso's son Denis Christel appears several times in the case files, singled out for his "exorbitant lifestyle" in a PNF document from 2019.

"The amount of property seized is at first glance very limited compared to the flood of embezzlement, but Denis Christel Sassou Nguesso is a first-rank player," said William Bourdon, a lawyer representing corruption watchdog Transparency International France, a civil plaintiff in the case.

"Given the weight of evidence, his denials are pathetic and insulting to the judges and to France," he added.

At least five members of the Nguesso family have come into investigators' sights since 2017.

Earlier this week, Syrian President Bashar al-Assad's uncle had a conviction upheld by France's top judicial court using a new law on ill-gotten gains introduced last year.

The Court of Cassation confirmed 85-year-old Rifaat al-Assad's sentence of four years in jail -- which he is unlikely to serve due to his age and ill health -- as well as confirming the confiscation of a 90-million-euro property empire.

The Republic of Congo is also called Congo-Brazzaville to distinguish it from its larger neighbour, the Democratic Republic of Congo.