Three suicide bombers who struck Brussels airport and a metro train in attacks claimed by the Islamic State were identified Wednesday, as the manhunt for a fourth suspect whose suitcase bomb failed to detonate intensified.

Prosecutors said brothers Ibrahim and Khalid El Bakraoui had carried out attacks at Zaventem airport and Maalbeek metro station, while bomb-making expert Najim Laachraoui was identified by police sources as the second airport bomber.

Authorities stepped up the manhunt for a third airport attacker, seen wearing a hat and white jacket on CCTV footage from Zaventem departure hall, whose explosive-packed suitcase failed to go off with the two other suicide bombers.

The three identified suspects behind the twin assaults, which killed 31 and injured 300, have been linked to the Paris attacks last November, underscoring the threat European nations face from the jihadist group.

Turkey said it had detained Ibrahim El Bakraoui near the Syrian border in June 2015 and deported him as a "foreign terrorist fighter", piling more pressure on Belgian authorities who have faced criticism for failing to tackle the extremist menace.

Prosecutor Frederic Van Leeuw revealed that Ibrahim had left a desperate "will" on a computer that he dumped in a trash can, in which he said he felt "hunted" and added "I don't know what to do".

In an apparent reference to Salah Abdeslam, the key suspect in the Paris massacre arrested in Brussels on Friday, Ibrahim added: "I don't want to end up in a cell next to him."

EU justice and interior ministers will convene Thursday in Brussels for an emergency meeting to work out a plan to address the threat posed by jihadists to Europe and the application of EU anti-terrorism laws across the bloc.

Leaders in Europe have reacted with outrage to the twin bombings, vowing to defend democracy and combat terrorism "with all means necessary".

Belgium authorities are under immense pressure over their apparent inability to smash domestic extremist networks, after it emerged that the Paris attacks were largely planned from the country.

Belgium is also Europe's top exporter of jihadist fighters to Syria per capita.

Belgian authorities had already been hunting the Bakraoui brothers, both Belgian nationals with long criminal records, over their links to Paris attack suspect Abdeslam.

They also issued a wanted notice for Laachraoui on Monday, the day before the attacks, with officials saying he had travelled to Hungary with Abdeslam last year and that his DNA was found on explosives linked to the Paris rampage.

Then on Wednesday, Turkish President Recep Tayyip Erdogan said Turkey had arrested one of the Brussels attackers last year and deported him to the Netherlands. A senior Turkish official later confirmed that it was Ibrahim El Bakraoui.

"Despite our warnings that this person was a foreign terrorist fighter, the Belgian authorities could not identify a link to terrorism," Erdogan said.

Belgium's Justice Minister Koen Geens denied however that the 30-year-old Belgian citizen had been flagged as a possible terrorist.

"At that time, he was not known here for terrorism," Geens told VRT television. "He was a common law criminal out on parole."

In a raid on Tuesday night investigators found a bomb factory in an apartment near where Ibrahim's computer was left in the Brussels district of Schaarbeek, an area that has links to Abdeslam.

They found 15 kilos (33 pounds) of TATP high explosive, chemicals and detonators, Van Leeuw said. Prosecutors said on Tuesday an unexploded bomb, an IS flag and bomb-making materials had been found.

Belgian Foreign Minister Didier Reynders said the attacks, the worst in the country's history had killed or wounded people of around 40 nationalities.

The dead include a Peruvian mother of twin girls, one of whom was injured by flying debris in the airport attacks.

The health ministry said late Wednesday that the number of people injured had climbed from 270 to 300, 61 of whom were in critical condition.

Brussels airport announced it would stay shut until at least Saturday while investigators continued to comb through the wreckage.

The city's subway system partially reopened, although rush-hour crowds were thinner than usual and soldiers were checking passengers' bags.

"I'm a bit afraid, especially for my little brothers," said 18-year-old Dominique Salazar as she took her siblings to school. "But we don't have any other choice to get around."

Belgium declared three days of mourning, and on Wednesday hundreds of airport staff and their families carried candles and flowers in a silent march and vigil near Zaventem.

"It could have happened to me," said security guard Gregory Lupant, adding he was worried about colleagues "who had not been heard from, and others who had lost a leg or finger."

King Philippe, Prime Minister Charles Michel and European Commission chief Jean-Claude Juncker led a minute's silence outside the EU headquarters in Brussels, the city that is also home to NATO, and mourners laid banners and candles in the city's Place de la Bourse.