PARIS (Reuters) - The French authorities have opened an investigation into a controversial comedian who mocked the killing of United States journalist James Foley and showed footage of it in a video.

Dieudonne M'Bala M'Bala said that decapitation "symbolises before anything progress, access to civilisation" in the video that appeared on the Internet last month. The Paris prosector's office said the police were starting a preliminary investigation into Dieudonne on the grounds that he had condoned terrorism.

In the video, called "Foley That Was", Dieudonne said the 2011 killing of former Libyan leader Muammar Gaddafi and that of former Iraqi president Saddam Hussein, who was hanged in 2006, never aroused the same indignation as Mr Foley's death. "The Rothschild mafia says no, that's ok, but James Foley, that's too much," Dieudonne says, an apparent but unexplained reference to the prominent Jewish banking family.

It also showed some footage of the killing. "I think decapitation symbolises before anything progress, access to civilisation," he said. "In France, we decapitated people in front of the masses, on the public plazas."

Mr Foley was beheaded by Islamic State militants last month after being kidnapped while covering the Syrian conflict and held for 20 months. The act caused outrage around the world.

Dieudonne, 46, has been repeatedly fined for hate speech in France where the local authorities in several towns have banned his shows as a threat to public order.

Although he says he is not anti-Semitic, the public authorities say he owes more than 65,000 euros (S$81,380) in fines related to past convictions for making anti-Semitic comments.

He is also credited with inventing the "quenelle", a gesture that critics have likened to an inverted Nazi salute and say carries anti-Semitic overtones.