Malfunctioning air traffic control systems at Amsterdam's Schiphol airport on Tuesday led to dozens of canceled flights and long delays at one of Europe's busiest transportation hubs.

The problems were resolved around 1600 GMT, but it would still take hours for operations to return to normal, a spokeswoman for Air Traffic Control the Netherlands said. She said it was still unclear what caused the problems, but excluded the possibility of a cyber attack.

Air France-KLM, the national Dutch carrier, said it had scrapped 50 flights. Delays ran up to more than four hours on other flights, a KLM spokesman said.

A spokesman for Schiphol could not give an exact number of cancellations and delays, but the airport's website showed problems with almost all incoming and outgoing afternoon flights.

On its web site, Eurocontrol, Europe's organization for air traffic control coordination and planning, showed a large number of flights to Schiphol were delayed more than 30 minutes. In a notice, it said airplanes could opt to divert to other airports.

A major computer malfunction in February crippled traffic at Schiphol for hours, causing delays or cancellations on more than 100 flights.

Schiphol received more than 63 million passengers in 2016, making it the third-busiest airport in Europe after London Heathrow and Paris Charles de Gaulle.