Paris and Los Angeles have stressed they are only interested in hosting the Olympics in 2024, amid suggestions the International Olympic Committee (IOC) could award the losing bidder the 2028 Games.
LA mayor Eric Garcetti said earlier this week that the city was competing for the 2024 Games "full stop", with the IOC reportedly keen to name the host for both events at a summit in Peru in September.
Both cities have now stressed that 2024 is their sole focus.
Paris co-chair Tony Estanguet told BBC Sport: "2024 is now or never for us.
"We believe we have the strongest offer but it is only available for 2024. We can't host the Games in 2028 because we don't have the project available for 2028.
"We have the guarantees, we have the public support, we have the political support, we have 95 per cent of existing venues.
"This is the fourth bid from Paris and 2024 is the centenary of the Games in Paris."
An LA 2024 statement quoted by the BBC added: "With all permanent venues already built and 88 per cent public support, only LA 2024 offers the lowest-risk and truly sustainable solution for the future of the Olympic movement in 2024 and beyond."
Proposed bids from Hamburg, Rome and Budapest have all fallen by the wayside since the process to find a host started.