The French government is insisting it will keep a plan to hold the 2024 Paris Olympics opening ceremony on the River Seine even after a deadly attack in the French capital at the weekend amplified existing security concerns.

Sports Minister Amelie Oudea-Castera told French radio that the plan could still be adapted within the idea of the river flotilla, as media reports indicated grave concern within the security forces that the ceremony could be vulnerable to attack.

A man known to the authorities as a radical Islamist with mental troubles on Saturday stabbed to death a German tourist close to the Eiffel Tower by the River Seine in what prosecutors are investigating as a suspected act of terror.

"There is no plan B, we have a plan A within which we have several alternatives," the minister told France Inter radio.

She said the "terrorist threat and in particular the Islamist threat exists" but added "it is not new and it is neither specific to France nor specific to the Games."

The plan is for a flotilla of boats to proceed along the Seine from Austerlitz bridge in the east of Paris to Iena bridge in the west carrying athletes, delegations and officials.

The culmination of the ceremony would then take place in front of the famed Trocadero Gardens across the river from the Eiffel Tower.

The idea has been championed by President Emmanuel Macron, who wants a unique spectacle that will leave a lasting imprint in the history of the Games.

But Oudea-Castera added that there were "a certain number of adjustment variables," notably the number of spectators who can attend, which will be decided in the spring and can be "modulated."

Also subject to adjustment could be "the number of events which will be authorised around the area and in Paris" on the sidelines of the ceremony and "the management of security perimeters."

Authorities initially planned to offer 500,000 free tickets for the ceremony as well as 100,000 paid tickets closest to the action, creating a gargantuan event with a total of 600,000 ticket holders and one of the largest spectator events in human history. (AFP)