Two explosions have been reported in the departures area of Zaventem airport in Brussels.
The Belgian fire service told local media there were at least several dead and wounded.
The cause of the explosions is unknown. The airport is being evacuated and has been closed to flights.
The blasts come four days after the capture in Brussels of Salah Abdeslam, the main suspect in the Paris attacks in November.
Images on social media showed smoke rising from one of the buildings, amid reports of panic as people fled the airport.
Rail transport to the facility has been halted and people have been told not to come to the airport.
Other reports say at least one of the explosions was close to the American Airlines check-in area but this has not been confirmed.
The Belgian broadcaster RTBF quoted a witness as saying there were people injured or unconscious in the departure area, opposite the Sheraton hotel.
Niels Caignau, a Swissport employee, told Flemish broadcaster VRT: "I was on a break and heard and felt a big explosion - we have from here a view over the departure hall and saw a plume of smoke come out.
"The windows are completely shattered. People went outside in shock. It doesn't look good."