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.”

Ryanair was among airlines that announced widespread disruptions, saying in a statement it canceled more than 400 flights affecting 70,000 passengers. The company said the strike affects all its flights over French airspace, as well as traffic in and out of French airports, and urged the European Union to reform air traffic rules.

One of the two unions leading the strike, UNSA-ICNA, said in a statement there are not enough employees to handle surging air travel and that inflation is eating away at salaries. The unions also are protesting new reform measures aiming to more tightly monitor their work, prompted by a near-collision at the Bordeaux airport.

Transport Minister Philippe Tabarot called the union demands — and their decision to strike just as French schools close for the summer and many families head on vacation — ″unacceptable.″