Two explosions have been heard at Brussels airport with video and stills pictures showing smoke rising from one of the terminal buildings.
The incident came as the Belgian capital was on a state of high alert following the arrest of Paris terror attack suspect Salah Abdeslam in the city last week.
Sky News Middle East correspondent Alex Rossi, who was at the airport en route for Tel Aviv, told the channel: "I could feel the buildings move."
According to reports the incident centred on an American Airlines desk in a departure hall.
Belgian media were reporting several casualties.
Mr Rossi told Sky News people were "dazed and shocked".
"The word is definitely two explosions.
"The thinking here by everybody is that it is some kind of terrorist attack although that hasn't been verified by anyone here at the airport.
"No word too of casualties. Don't know how the explosion took place, the method if you like. But it certainly seems Brussels airport has been targeted in a terrorist attack.
"We are all being moved out of the airport now towards the emergency exit. There is a great deal of confusion here. Certainly there are a number of very upset, as you might imagine, very frightened people."
He added: "There are fears that there might be other attackers."
Images on social media showed shattered windows and smoke rising from an airport building.
Video footage shot from an airport car park showed dozens of people fleeing in terror.
Abdeslam, suspected as a planner in the attacks that killed 130 people in Paris, was arrested on Friday after a four-month manhunt, in the same neighbourhood in Brussels where he grew up.
But the Belgian authorities fear he had accomplices while on the run who are still at large and could pose a threat.
Belgian prosecutor Frederic Van Leeuw told reporters at a news conference in Brussels on Monday: "(It's clear) we have a general threat."
Mr Van Leeuw said investigators hope to find out the details of Abdeslam's actions between the November 13 attacks and his arrest, "if he decides to tell us".
Abdeslam, 26, a French citizen who grew up in Brussels' Molenbeek neighbourhood, slipped through police fingers on several occasions, including the day after the attacks.
He was interviewed three times on Saturday, the day after his capture - once by prosecutors and twice by an investigating judge - and "wasn't in great shape" because he had been shot in the leg by police during his capture, Mr Van Leeuw said.
Belgian prosecutors appealed to the public on Monday for information about a man who allegedly travelled to Hungary last year with the top suspect in the Paris attacks.
The federal prosecutor's office said in a statement they are seeking details about 24-year-old Najim Laachraoui, who is said to have travelled to Syria in February 2013. It said Laachraoui was checked by guards at the Austria-Hungary border while driving in a Mercedes with Abdeslam and one other person.
Laachraoui is said to have rented a house under the name of Soufiane Kayal in the Belgian town of Auvelais that was allegedly used as a safe house. Prosecutors said traces of his DNA were found there.
Laachraoui is "someone who must explain himself," the prosecutor said, stressing that "clues" do not amount to proof.
Abdeslam has a court hearing on Wednesday. France has requested his extradition but Abdeslam's lawyer says his client will fight the request.