Belgium yesterday said two brothers with links to the Paris attacks were among the suicide bombers who struck Brussels, as mourners observed a minute's silence for the victims of the carnage.

Hundreds of people gathered in a historic city square applauding and chanting "We love Belgium" in an emotional tribute to the 31 people killed and 270 injured in Tuesday's blasts at Brussels airport and a metro train.

Prosecutors identified Ibrahim El Bakraoui as one of two men who blew themselves up in the Zaventem airport departure hall while his brother Khalid struck at the Maalbeek metro station in the attacks on the symbolic heart of Europe.

Police stepped up a manhunt for a third airport assailant whose bomb failed to go off in the attacks claimed by the Islamic State group which have left European leaders once more grappling for ways to tackle the jihadist threat.

Belgian authorities had already been hunting the Bakraoui brothers, both Belgian nationals with long criminal records, over their links to Salah Abdeslam, the key suspect in the Paris massacre who was arrested in Brussels on Friday after four months on the run.

Federal prosecutor Frederic Van Leeuw revealed that airport bomber Ibrahim El Bakraoui had left a desperate "will" on a computer that he dumped in a trash can in which he said "I don't know what to do."

In an apparent reference to Abdeslam, Bakraoui added: "I don't want to end up in a cell next to him."

Analysts say the police hunt seems to have prompted the bombers to rush into an attack in Belgium after months of lying low, according to the testament found on the laptop.

A third man in a hat and white jacket, seen on CCTV footage with Bakraoui and another unidentified suicide attacker pushing their bomb-filled bags through the departure hall shortly before the attacks, "is on the run," Van Leeuw said.

Belgian media withdrew a report that a man arrested in the capital on Tuesday was Najim Laachraoui, another suspect whose DNA has been found on explosives linked to the Paris rampage.

Analysts say Laachraoui is believed to be a key bomb maker, and French media say he also played a major role in the terror attacks in Paris.

Meanwhile, Turkey yesterday said Ibrahim was detained by Turkish officials on the border with Syria in June 2015. They deported him with the warning that he was a "foreign fighter" but the Belgian authorities let him go. Belgium has not yet responded to the claims.

Investigators found a virtual bomb factory in an apartment near where Ibrahim's computer was left, during a raid in the Brussels district of Schaerbeek on Tuesday night, 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.

Three days of national mourning was declared in a country deeply shocked by the bloodshed. 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.

In the city's Place de la Bourse where mourners laid banners and candles, defiant applause broke out among the large crowd gathered to honour the dead, chanting: "Long live Belgium".

Belgian Foreign Minister Didier Reynders said around 40 nationalities were among the dead and wounded.

The dead include a Peruvian mother of twin girls, one of whom was also injured by flying debris in the airport attacks, and a Moroccan woman killed in the metro blast, while a Briton is missing. The wounded include citizens of Britain, Colombia, France and the United States.

"It's war... it's the kind of trauma seen in war," said a doctor at a hospital which treated some of the injured victims.

"Limbs torn off, impacts from flying glass and metal shrapnel -- either from the bomb or, for example, furniture -- head trauma, vascular lesions and fractures," said Jacques Creteur, head of the intensive care unit at Erasme hospital.

Authorities are under immense pressure over their apparent inability to smash jihadist networks in Belgium, Europe's top exporter of jihadist fighters to Syria per capita.

Broadcaster RTBF said Khalid El Bakraoui had rented an apartment in Brussels last week under a false name where Abdeslam's fingerprints were found. He is also linked to another apartment in southern Belgium that Abdeslam and other jihadists used before the Paris attacks.

Leaders across Europe have reacted with outrage to the Brussels bombings, with the EU calling an emergency meeting of interior ministers and vowing to defend democracy and combat terrorism "with all means necessary".

Cities across the globe have scrambled to boost security at airports and other potential targets, while Belgium remains on maximum alert.

Australian Prime Minister Malcolm Turnbull yesterday blamed Europe's porous borders and lax security for the attack. Turnbull said that while it was up to Europe to set its policies, his nation's border protection measures and domestic security arrangements "are much stronger than they are in Europe, where regrettably they allowed security to slip".

Security was also tighter at airports around Asia, with South Korea, Japan, Indonesia, Thailand and India saying that they were deploying additional resources at the major hubs.

Analysts said the attacks pointed to a sophisticated jihadist network in Europe, and French Prime Minister Manuel Valls said there was an "urgent need" to tighten the EU's external borders following the attacks.

The Islamic State said the bombings were carried out by "soldiers of the caliphate" against "the crusader state" of Belgium -- part of the international coalition waging strikes against IS in Iraq.