Two explosions tore through Brussels airport on Tuesday morning killing at least 21 people in what Belgian public broadcaster VRT called a suicide attack, and a further blast struck a metro station in the capital shortly afterwards.


French media reports that the border between Belgium and France is sealed.


The Belga agency said shots were fired and there were shouts in Arabic shortly before two blasts rang out at the airport. Pictures on social media showed smoke rising from the terminal building through shattered windows and passengers running away down a slipway, some still hauling their bags.


All public transport in Brussels was shut down, as it was in London during 2005 militant attacks on the underground that killed 52.


The Belgian Crisis Centre, clearly wary of a further incident, appealed to the population: "Stay where you are".The blasts at the airport and metro station occurred four days after the arrest in Brussels of a suspected participant in November militant attacks in Paris that killed 130 people.


Belgian police had been on alert for any reprisal action.British Sky News television's Alex Rossi, at the airport, 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."


The blasts triggered concern across western Europe with Britain calling a meeting of its crisis response committee. The Dutch military strengthening security at airports and borders and London's major airports Heathrow, Gatwick, Stansted and City said they were working to provide a high police presence.


Video showed devastation inside the departure hall with ceiling tiles and glass scattered across the floor. Some passengers emerged from the terminal with blood spattered over their clothes. A witness said the blasts occurred at a check-in desk.


The metro station hit by the explosion was Maelbeek, close to European Union institutions. There were no details immediately available of casualties in this second incident of the morning.VRT said the blast went off on a carriage of the underground train but that no one was killed.


Earlier, private broadcaster VTM had said there were reports of the metro death toll at 10.Other media reports cited the transport authority saying the explosion was on a train travelling between Schuman and Maelbeek stations, close to the EU instutions.


A local journalist tweeted a photograph of a person lying covered in blood among smoke outside Maelbeek metro station, on the main Rue de la Loi avenue which connects central Brussels with the EU institutions.