Footage and images released on social media showed rising smoke on the terminal, as well as people fleeing the area with panic.
Causalities have not been confirmed yet, and the reason of the explosion is unknown. The airport has been evacuated and closed to flights, while train transportation to the airport has been halted, according to local media.
Shots were fired and a person allegedly cried in Arabic right before the attack, according to several eye witnesses confirmed by Belgian media.
"Avoid the airport area. Flights have been cancelled" said the statement of Brussels Airport on Twitter.
The blast erupted after Turkish Airlines passengers entered the plane for the Brussels-Istanbul flight.The plane was not affected by the explosion.
Passengers have been told to wait at the apron.
Belgium raised "terror alert" to 4, the maximum level, said Belgium Interior Minister Jan Jambon.
The cabinet is expected to hold an emergency meeting after the attack.
Salah Abdeslam, the main suspect of the deadly Paris terror attacks in November was caught in Brussels four days ago.