European Union president Donald Tusk condemned a series of blasts early Tuesday at Brussels airport and a metro station as appalling "terrorist attacks".

"I am appalled by the bombings this morning at Zavantem airport and the European district in Brussels which have cost several innocent lives and injured many others," Tusk said in a statement, with the death toll standing so far at 21.

"These attacks mark another low by the terrorists in the service of hatred and violence."