Dutch Airline KLM has announced it was temporarily stopping ticket sales for most of it flights from Amsterdam Schiphol Airport through Sunday, due to the airport’s ongoing crowding issues caused by staff shortages.

In a statement Thursday, the airline said it is taking the action to guarantee seats for customers whose flights had been cancelled due to the long security lines at Schiphol. The airline said the restrictions do not apply to premium bookings.

Air France-KLM spokesperson Gerrie Brand said Thursday, “KLM is putting a brake on ticket sales for flights leaving up until and including Sunday because Schiphol can’t get its security problems fixed.” Amsterdam is KLM’s hub city, and it is the largest airline serving the airport.

Schiphol, one of Europe’s busiest airports – has been experiencing extremely long security lines in recent weeks due to a shortage of security personnel, as well as labor issues earlier in this year.

Lines routinely run out of the building and onto the street with customers reporting wait times as long as six hours, resulting in missed flights. Media reports say Monday alone more than 500 flights were delayed from Schiphol, while over 50 were cancelled.

Airport officials said Thursday they are working to recruit more security staff before the summer holidays begin, while it would also work with airlines to guarantee better planning of flights during the busiest weeks.

The airport said it was also in talks with unions about higher wages for security personnel.

Some information for this report was provided by the Reuters news agency.