Air France on Wednesday said it would only carry out three in four planned flights on Thursday and warned of travel delays and disruptions after worker unions called a strike over pay.

Only half of the long-haul flights leaving Paris on Thursday would be going ahead, according to the airline's forecast, which said it would cover 75 percent of middle distance flights to and from the city's Charles de Gaulle airport.

Disruptions to short flights would be less onerous, though the airline forecast it would cover only 85 percent of those. In all, the strikes would hit a quarter all Air France flights.

"Disruptions and delays cannot be excluded," Air France added in a statement.

Worker unions representing Air France pilots, cabin crew and ground staff called the strike amid demands for a 6 percent pay rise, which the airline has refused to meet.