Several people were killed in the explosions that tore through the departure hall of Brussels airport on Tuesday, said Belgium's public broadcaster RTBF.
The Belgian news agency Belga said shots were fired and there were shouts in Arabic shortly before the explosions.
The blasts come just four days after the arrest in Brussels of Salah Abdeslam, the main suspect in November's Paris terror attacks that killed 130 people and the Belgian police had been on high alert.
Belgian media said rail traffic to the airport was suspended, flights have been cancelled and the airport was evacuated. There was no immediate official comment on the cause of the
attack.
Social media showed pictures of smoke rising from the departure hall of the airport, where all windows were shattered by the blast. Passengers were seen running away down a slipway from the departure lounge.
Sky News television's Alex Rossi, at the scene, said he heard two "very, very loud explosions".
"I could feel the building move. There was also dust and smoke as well...I went towards where the explosion came from and there were people coming out looking very dazed and shocked."
"The thinking here is that it is some kind of terrorist attack - that hasn't been verified by any of the authorities here at the airport."