BRUSSELS: Belgian investigators were on Wednesday hunting a man seen fleeing Brussels airport after a suicide bombing, as the country held three days of mourning for around 35 people killed in twin attacks claimed by the militant Islamic State (IS).

Two massive suicide blasts by men with bombs in their bags hit Zaventem Airport on Tuesday morning, leaving blood and mangled bodies strewn across the check-in hall and sending terrified travellers fleeing.

At around 9am local time, an hour after the airport blasts, 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.

Belgian authorities launched a huge manhunt after the attacks, releasing pictures of two of the suspects pushing trolleys with their bombs through the terminal and a third man whose explosives did not go off.

Belgian state broadcaster RTBF claimed to have identified the two attackers, but this information was not verified by Belgian authorities.

The report Wednesday said the brothers were known to police for past crimes, but nothing relating to terrorism.

Police helicopters hovered over the city late into the night and prosecutors said raids were carried out across Belgium, adding that a bomb, an IS flag and chemicals had been found in one apartment.

The fact that extremists were able to hit high-profile targets in Brussels, capital of the European Union, just months after IS militants killed 130 people in Paris, will raise fresh questions about the continent's ability to prevent terrorism.

It also underscores doubts about how Belgium has allowed extremism to develop unchecked, coming days after key Paris attacks suspect Salah Abdeslam was arrested in Brussels following four months on the run.

"This is a day of tragedy, a black day," Prime Minister Charles Michel said, vowing the country would not be cowed by the "deadliest attacks we have ever seen in Belgium".

Belgium in mourning

Flags will fly at half mast at public buildings across Belgium for three days of national mourning through Thursday, after crowds gathered in the Place de la Bourse square to sing songs and lay flowers in memory of the dead.

"People were just going to work, to school and they have been cut down by the most extreme barbarity," Michel said. "We will continue to protect liberty, our way of life."

IS claimed the bombings, saying "soldiers of the caliphate" had carried out the attacks against "the crusader state" of Belgium.

Leaders across Europe reacted with outrage, with the EU vowing to combat terrorism "with all means necessary" on a continent that has been on high alert for months.

"The whole of Europe has been hit," said French President Francois Hollande, whose country is still reeling from November's attacks.

Landmarks from the Eiffel Tower in Paris to Berlin's Brandenburg Gate were lit up in the black, yellow and red of Belgium's national flag in solidarity, while social media users shared images of beloved Belgian cartoon character Tintin in tears.

Thousands also posted tweets using the #StopIslam handle to express their anger against Muslims after the attacks, although others rejected it as Islamophobic and racist.

US President Barack Obama vowed to stand with Belgium in the face of the "outrageous" attacks, while the FBI and New York police said they would send investigators to help.

Hundreds of flights and trains were cancelled as Europe tightened security, while the US warned citizens about the “potential risks” of travelling in Europe and New York and Washington stepped up security.

Bombs in their bags

Pierre Meys, spokesman for the Brussels fire brigade, told AFP at least 14 people had been killed at the airport, while Brussels mayor Yvan Mayeur said “around 20” died in the metro.

The first to be identified was Adelma Marina Tapia Ruiz, a Peruvian woman who had been living in Brussels for six years and was with her family in the airport when the blast went off, according to the foreign ministry.

More than 200 people were wounded in the two attacks, including four Mormon missionaries ─ three Americans and one French ─ two Britons, two Colombians and an Ecuadorian. France said eight of its nationals were hurt, though it was unclear if this included the Mormon.

Anaysts said the attacks pointed to a sophisticated militant network operating in Europe, and Australia's Prime Minister Malcolm Turnbull said Europe had "allowed security to slip", as he questioned the EU's Schengen passport-free zone.

Belgian authorities published surveillance images showing the three male suspects of the airport attack. Two had dark hair and were wearing a glove on only one hand, and a third wearing a hat and a white coat.

“They came in a taxi with their suitcases, their bombs were in their bags," Zaventem mayor Francis Vermeiren said.

"They put their suitcases on trolleys, the first two bombs exploded. The third also put his on a trolley but he must have panicked, it didn't explode."

Belgian authorities had been on alert after Abdeslam, Europe's most wanted man, told investigators he had been planning an attack on Brussels.

He was caught on Friday after evading capture for four months including, officers believe, escaping a shootout that saw a Kalashnikov-wielding man killed and four police officers wounded.