BRUSSELS - Belgium said on Wednesday that two brothers with links to the Paris attacks were among the suicide bombers who struck Brussels, as mourners observed a minute's silence for 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."

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.

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 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 have been 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 have 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. An American Mormon missionary who was wounded had, by a twist of fate, had close calls in the Boston Marathon bombings and Paris attacks in November that left 130 people dead. "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.

Brussels airport announced it would stay shut Thursday while investigators continued to comb through the debris left by the bombings that wrecked its main terminal building.

While the city's subway system partially reopened after a day of lockdown, Belgium's friendly international football match against Portugal, due to take place in Brussels next Tuesday, was moved to Portugal.

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

Soldiers were checking passengers' bags at metro stations and rush-hour crowds were thinner than usual.

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".

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.

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

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.

Second Brussels airport bomber identified

The second suicide bomber who blew himself up at Brussels airport has been identified as Najim Laachraoui, police sources told AFP on Wednesday.

Moroccan-born Laachraoui's DNA was found on explosives used in the November 13 Paris attacks as well as at an apartment in Brussels where bomb-making equipment and a fingerprint from Salah Abdeslam - prime suspect in the Paris attacks - had been found in December.

Prosecutors have said Laachraoui "travelled to Syria in February 2013," and was registered under a false name at the border between Austria and Hungary last September.

He was travelling with Franco-Moroccan Abdeslam and Algerian Mohamed Belkaid, who was killed in a March 15 Brussels raid, three days before Abdeslam was captured.