Belgium authorities are working to secure "other sites" that are of concern in the wake of two attacks in the capital Brussels, Prime Minister Charles Michel says.
Military reinforcements have been deployed and controls are being carried out at Belgium's borders, he adds.
"In this moment of tragedy, in this dark moment for our country, more than ever I want to call on everyone to display calm, but also solidarity. We are confronted with an ordeal, a difficult ordeal and we have to face this ordeal by being united," Michel says.
The premier calls the attacks "blind, violent and cowardly."