Two huge blasts, both possibly caused by a suicide bomber, hit the check-in hall at Zaventem Airport, strewing the scene with blood and mangled bodies and sending hundreds of terrified travellers fleeing in terror.

Belgian authorities released pictures of two of the suspected suicide attackers pushing trollies through the terminal and said they were "actively searching" for a third whose bomb failed to go off.

The fact that extremists were able to hit high-profile targets in Brussels, Europe's symbolic capital, just months after IS militants killed 130 people in Paris, will raise fresh questions about the continent's ability to cope with the terror threat.

It also underscores doubts about how Belgium has allowed extremism to develop unchecked, coming just four days after the dramatic arrest in Brussels of key Paris suspect Salah Abdeslam after four months on the run.

"This is a day of tragedy, a black day," Belgian Prime Minister Charles Michel said and announced three days of national mourning after the "deadliest attacks we have ever seen in Belgium".

Belgian King Philippe condemned the "cowardly and odious" assault. The Islamic State claimed the bombings, saying "soldiers of the caliphate" had carried out the attack against "the crusader state" of Belgium.

Hundreds of flights and trains were cancelled yesterday as security across Europe was tightened after the bombings, which Michel branded "blind, violent and cowardly".

But he insisted Belgium would not be cowed. "People were just going to work, to school and they have been cut down by the most extreme barbarity," Michel told a news conference. "We will continue to protect liberty, our way of life."

About an hour after the airport blasts at around 8:00 am (0700 GMT), a third explosion rocked Maalbeek metro station, in the heart of the city's EU quarter, just as commuters were making their way to work.

The city is the headquarters of both NATO and the European Union. Belgian authorities published surveillance camera images showing three male suspects pushing trollies with suitcases past the check-in area. Two have dark hair and were both wearing a glove on only one hand, and a third, being hunted by Belgian police, is wearing a hat and a white coat.

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