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.
"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.
Passengers have been told to wait at the apron.
Salah Abdeslam, the main suspect of the deadly Paris terror attacks in November was caught in Brussels four days ago.