PARIS (AP) — About 40% of flights were canceled Friday at all Paris airports and tens of thousands of passengers were rearranging plans at the height of the summer travel season because of a strike by French air traffic controllers seeking better working conditions.

Disruptions started hitting airports around France on Thursday and intensified Friday. The national civil aviation authority asked airlines to cancel 40% of flights Friday at Charles de Gaulle, Orly and Beauvais airports serving Paris, half of flights in Nice and 30% of flights in Marseille, Lyon and some other cities.

Despite the preventive cancelations, the authority warned in a statement that ″disruptions and long delays are to be expected at all French airports.”