Three weeks after being struck by Islamic State group (IS) suicide bombers, Brussels Airport was forced to close again on Tuesday due to industrial action by Belgian air traffic controllers, officials said.

The airport, one of Europe's busiest, had reopened on April 3 with very limited capacity. Other Belgian airports had taken on many connections but at least some of these, including nearby Charleroi, were affected by the controllers' action, which is part of a months-long dispute over employment conditions.

"Currently no air traffic control is possible at Brussels Airport," Brussels Airport said in a statement. "This means that there are no inbound or outbound flights."

Transport authorities said police had shut down roads around the airport, which has been the only means of access for passengers arriving or departing as the railway station has remained closed since the attacks. Along with a suicide bombing on the city's metro, the bombers killed 32 people. Police have arrested several local men accused of plotting the attacks.

Belgocontrol, the agency which handles Belgian airspace, said it was seeking a solution with the trade union. Labour leaders asked staff to call in sick on Tuesday due to disagreements over a proposed dispute settlement, including a rise in the minimum retirement age.

Brussels Airlines, the main Belgian carrier whose flights have accounted for the bulk of those operating from Brussels this month, said on its website: "As from 17:00 local time Brussels (1500 GMT), flight disruptions are to be expected.

"At this moment the situation is very unclear, we continuously monitor the situation and will inform you about any flight cancellation as soon as we have more news. Our first priority is to limit the impact on our flight operations and to get our guests as quickly as possible to their destination."

At Charleroi airport, also known as Brussels South, spokesman Vincent Grassa said: "We are indeed affected by the strike ... There can be landings but no takeoffs."