Belgium in mourning after dozens killed and injured in terror attacks

About 40 minutes later, another blast struck subway commuters in central Brussels near the Maelbeek station, which sits amid the European Commission headquarters.

Authorities released a photo taken from closed-circuit TV footage of three men pushing luggage trolleys, saying two of them apparently were the suicide bombers and that the third - dressed in a light-coloured coat, black hat and glasses - was at large. They urged the public to contact them if they recognised him. The two men believed to be the suicide attackers apparently were wearing dark gloves on their left hands.

In police raids across Brussels, authorities later found a nail-filled bomb, chemical products and an Islamic State flag in a house in the Schaerbeek neighbourhood, the state prosecutors' office said in a statement.

European security officials have been braced for a major attack and warned that IS was actively preparing to strike.

The arrest on Friday of Salah Abdeslam, a key suspect in the Paris attacks last November, heightened those fears, as investigators said many more people were involved than originally thought and that some are still on the loose.

"In this time of tragedy, this black moment for our country, I appeal to everyone to remain calm but also to show solidarity," said Belgian Prime Minister Charles Michel, who announced three days of mourning in his country's deadliest terror strike.

"Last year it was Paris. Today it is Brussels. It's the same attacks," said French President Francois Hollande.

Belgium raised its terror alert to the highest level, shut the airport and ordered a city-wide lockdown, deploying about 500 soldiers onto Brussels' largely empty streets to bolster police checkpoints. France and Belgium both reinforced border security.

Medical officials treating the wounded said some victims lost limbs, while others suffered burns or deep gashes from shattered glass or suspected nails packed in with explosives.

Among the most seriously wounded were several children.

The bombings came barely four months after suicide attackers based in Brussels' Molenbeek district slaughtered 130 people at Paris nightspots, and intelligence agencies had warned for months a follow-up strike was inevitable. Those fears increased following Abdeslam's arrest in Molenbeek, along with police admissions that others suspected of links to the Paris attacks were at large.

Hundreds gather in Brussels square to show defiance after bomb attacks

A high-level Belgian judicial official said a connection by Abdeslam to Tuesday's attacks is "a lead to pursue".

Abdeslam has told investigators he was planning to "restart something" from Brussels, said Belgian Foreign Minister Didier Reynders. He said on Sunday that authorities took the claim seriously because "we found a lot of weapons, heavy weapons in the first investigations and we have seen a new network of people around him in Brussels".

While they knew that some kind of extremist act was being prepared in Europe, they were surprised by the size of Tuesday's attacks, said Belgian Interior Minister Jan Jambon.

"It was always possible that more attacks could happen, but we never could have imagined something of this scale," he said.

Officials at the airport in the Brussels suburb of Zaventem said police had discovered a Kalashnikov assault rifle and an explosives-packed vest abandoned at the facility, offering one potential lead for forensic evidence. Bomb disposal experts safely dismantled that explosive device.

A US administration official said American intelligence officers were working with European counterparts to try to identify the apparently skilled bomb-makers involved in the Brussels attacks and to identify any links to bombs used in Paris.

The official said that at least one of the bombs at the airport was suspected to have been packed in a suitcase left in the departures hall.

Three intelligence officials in Iraq told the AP that they had warned European colleagues last month of IS plans to attack airports and trains, although Belgium was not specified as a likely target.

One of the officials said Iraqi intelligence officials believe that three other IS activists remain at large in Brussels and are plotting other suicide-bomb attacks.

Leaders of the European Union said in a joint statement that Tuesday's assault on Brussels "only strengthens our resolve to defend European values and tolerance from the attacks of the intolerant".

Reflecting the trauma of the moment, Belgian officials offered uncertain casualty totals at both the airport and subway, where police conducted controlled explosions on suspicious abandoned packages that ultimately were found to contain no explosives.

Belgium's health minister, Maggie de Block, said 11 people were killed and 81 injured at the airport, where thousands of passengers were waiting to check luggage and collect boarding cards.

Brussels Mayor Yvan Mayeur said 20 people died and more than 100 were wounded in the subway blast. Rescue workers set up makeshift first aid centres in a nearby pub and hotel.

Passengers on other trains said many commuters were reading about the airport attacks on their smartphones when they heard the subway blast. Hundreds fled from stopped trains down tunnel tracks to adjacent stations.

Political leaders and others around the world expressed their shock at the attacks.

"We will do whatever is necessary to support our friend and ally Belgium in bringing to justice those who are responsible," US President Barack Obama said.

Belgium's king and queen said they were "devastated" by the violence, describing the attacks as "odious and cowardly".

After nightfall, Europe's best-known monuments - the Eiffel Tower, the Brandenburg Gate and the Trevi Fountain - were illuminated with Belgium's national colours in a show of solidarity.