Lionel Cironneau, Associated Press
A passenger walks past check-in counters during the third day of a French airline strike in the Nice airport, southeastern France, Wednesday, Feb. 8, 2012. Air France canceled up to 40 percent of its long-distance flights Wednesday because of a strike by pilots and other personnel over their right to hold unannounced walkouts.

PARIS — France's leading pilots' union threatened Thursday to extend a strike that has canceled hundreds of flights worldwide and cost Air France tens of millions of euros (dollars).

Passengers faced a fourth day of cancellations at French airports on Thursday because of the strike. Pilots, cabin and ground crews are protesting a draft law that would forbid workers from holding unannounced walkouts.

Air France said it was canceling up to 35 percent of long-distance flights worldwide and 25 percent of other flights Thursday because of the strike. Paris' busy Charles de Gaulle airport saw the most flights canceled.

The strike is set to end Thursday night, and unions meet with the Transport Ministry on Friday to discuss their complaints about the bill.

Depending on how those talks go, unions may resume striking in the coming days, said Yves Deshayes, head of the SNPL pilots' union.

"Tonight we will suspend the conflict. Tomorrow morning at the meeting at the Transport Ministry, we will allow the opportunity for labor dialogue to take place, before eventually resuming this strike," he told reporters at Charles de Gaulle airport.

The airport was generally calm and less busy than usual Thursday because of all the canceled flights. A group of puzzled Japanese tourists waited in one terminal, trying to figure out when they would make it home after their flight was annulled.

Air France said it has been losing €8 million to €10 million ($10.6 million to $13.3 million) a day because of the strike.

The airline recommended that passengers postpone any flights planned through Friday. Flights operated with partner airlines in the Skyteam alliance have also been affected by the cancellations, an official with the Paris airport authority said.

Angela Charlton in Paris contributed to this report.