An air traffic control technical failure closed Belgian airspace Wednesday morning, which caused hundreds of flights to be either canceled or diverted.

The problem began at 0945 local time Wednesday morning. Eurocontrol recently tweeted there will be a “gradual restarting of systems that will start at 12UTC with some flights to/from Brussels.”

As of 1 p.m. local time, Brussels Airlines had to cancel 85 flights, including four long-haul flights to Africa,” airline spokesperson Wencke Lemmes told ATW in Brussels.

She added that more than 40% of planned flights for Wednesday had to be canceled; seven so far have been diverted to other airports. “This affects more than 11,000 passengers from Brussels Airlines. We do hope, that we can operate several other long haul flights, like to New York JFK, later during the day,” Lemmes said.

Until noon, Zaventem had to cancel 109 flights; 25 had been diverted to other airports around Europe, including Frankfurt, Paris and Maastricht Aachen.

No exact details had been given what is the cause of the technical failure.

The airspace closure does not impact overflights above FL245 (approximately 24,500 feet) as these are controlled by the Maastricht Upper Area Control Centre, which is unaffected.