At least 31 people have been killed and many seriously injured in attacks at Brussels international airport and a city metro station. Photo: Horst Pilger

Twin blasts hit Zaventem airport at 07:00 GMT, killing 11 and injuring 81, Belgium's health minister said.

Another explosion struck Maelbeek metro station an hour later. Twenty people were killed, Brussels mayor Yvan Mayeur said.

Belgium has now raised its terrorism threat to its highest level.

The attacks come four days after Salah Abdeslam, the main fugitive in the Paris attacks, was seized in Brussels.

Prime Minister Charles Michel called the latest attacks "blind, violent and cowardly", adding: "This is a day of tragedy, a black day... I would like to call on everyone to show calmness and solidarity".

European Council president Donald Tusk said: "These attacks mark another low by the terrorists in the service of hatred and violence."

US President Barack Obama called the blasts "outrageous attacks against innocent people".

The airport and the entire public transport system in Brussels are closed.

'Blood everywhere'

Two blasts tore through the departures area of Zaventem airport shortly after 08:00 local time (07:00 GMT).

The Belgian prosecutor said "probably a suicide bomber" was involved.

The state-owned Belga news agency reports that shots were fired and shouts in Arabic were heard before the two explosions.

Public broadcaster VRT said an assault rifle was found next to a dead attacker. Private broadcaster VTM added that an unexploded bomb belt had been found. Reports say it will be detonated safely soon.

Zach Mouzoun, arriving on a flight from Geneva, told France's BFM television: "It was atrocious. The ceilings collapsed. There was blood everywhere, injured people, bags everywhere. We were walking in the debris. It was a war scene."

All flights have been diverted. Eurostar has cancelled all trains to and from Brussels. The Thalys France-Benelux train operator says the entire network is closed.