Belgium's health minister Maggie De Block said earlier the toll was 31 dead and 250 injured.

Police are seeking information on this man in relation to the Brussels attacks. Credit:VTM News screenshot

Police in Belgium have already issued a wanted notice for a man suspected of involvement. They issued a photograph of a man, dressed in a white shirt and jacket and wearing a dark hat, as he pushed a luggage trolley through the airport before the blasts occurred.

Two men in black, who were photographed pushing trolleys laden with luggage next to the man in white, are believed to have been suicide bombers killed in twin blasts, according to the federal prosecutor.

A government official said the third suspect had been seen running away from the airport building. Local media said police had found an undetonated suicide vest in the area.

In an interview with local broadcaster VTM, Ms De Block said 11 people were killed in explosions at Brussels Airport, which occurred just before 8am local time, and about 20 people had died in an attack on the Maalbeek metro station, which occurred about an hour later.

Nam Laachraoui, left captured on CCTV, moments before the blast with two other bombers. Credit:AP

Belgian authorities were still checking whether the attacks were linked to the arrest of Salah Abdeslam, according to Federal Prosecutor Frederic Van Leeuw, although US officials said the level of organisation involved suggested they had previously been in preparation.

The wanted notice issued for the man in white on Tuesday follows a similar call issued by the investigating judge on Monday for a man named Najim Laachraoui, 25, who had travelled to Hungary in September with Abdeslam.

Two explosions were heard at Brussels airport. Credit:London Telegraph

Laachraoui has since emerged as a key suspect in the Brussels attacks, and is now one of Europe's most wanted men.

The poor quality of the images on the two wanted notices left it unclear whether the man in the white shirt at the airport could be Laachraoui, whose picture was on the front pages of Belgian newspapers on Tuesday morning.

Brussels airport was rocked by at least two explosions, according to witnesses.

The three suspects captured on CCTV travelled to the airport by taxi, according to local mayor Francis Vermeiren.

"They came in a taxi with their suitcases, their bombs were in their bags," he told AFP.

An injured person at Brussels airport. Credit:Twitter: Ketevan Kardava

"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." He said bomb disposal experts later blew up the suitcase that didn't explode.

Mr Vermeiren said the airport was "a war zone, atrocious to see, atrocious to live through. But strength and solidarity have won".

Reuters reported three people have been arrested by German police in a car with Belgian licence plates near the border with Austria.

The three suspects from Kosovo were arrested before the attacks that killed at least 30 people at a metro station and the international airport in Brussels, a police spokesman said.

Police made the arrests on a motorway in the southern state of Bavaria after receiving an intelligence tip-off.

"Investigations have been launched into the suspected planning of a serious criminal act against the state because there was notification of that," the spokesman said.

He said there was no indication so far that the three suspects had any links to the attacks in the Belgian capital, adding that this could not be ruled out.

Last week, explosives and an Islamic flag were found during a raid on a Brussels flat. Police also found a fresh fingerprint of Abdeslam's there, putting police on his trail.It was not clear if Abdeslam had been involved at that stage in the airport attack plan.

A bomb and an Islamic State flag were also found later on Tuesday in a flat in Brussels.

A witness said he heard shouts in Arabic and shots shortly before two blasts struck a packed airport departure lounge at Brussels Airport.

US President Barack Obama led calls of support to Belgian Prime Minister Charles Michel after Brussels went into a state of virtual lockdown.

"We must be together regardless of nationality or race or faith in fighting against the scourge of terrorism," Obama told a news conference in Cuba. "We can and we will defeat those who threaten the safety and security of people all around the world."

Prime Minister Michel spoke at a Brussels news conference of a "black moment" for his country. "What we had feared has come to pass."

Belgian police and combat troops on the streets had been on alert for reprisals after Abdeslam's arrest but the attacks took place in crowded areas where people and bags are not searched.

Police operations were under way at several points in the city but a lockdown imposed immediately after the attacks was eased in the evening, and commuters and students headed home as public transport partially reopened.

Islamic State issued a statement claiming responsibility: "We promise the crusader alliance against the Islamic State that they will have black days in return for their aggression against the Islamic State," the jihadist group said.

Belgium, home to the European Union and the headquarters of the NATO military alliance, has sent warplanes to take part in operations against Islamic State in the Middle East.

'It was massive'

Austrian Horst Pilger, who was awaiting a flight with his family when the attackers struck, said his children had thought fireworks were going off, but he instantly knew an assault was underway.

"My wife and I both thought 'bomb'. We looked into each other's eyes," he told Reuters. "Five or 10 seconds later there was a major, major, major blast in close vicinity. It was massive."

Pilger, who works at the European Commission, said the whole ceiling collapsed and smoke flooded the building.

Security services found and destroyed a third bomb after two blasts at the airport.

Video showed devastation in the hall with ceiling tiles and glass scattered across the floor. Some passengers emerged from the terminal with blood spattered over their clothes. Smoke rose from the building through shattered windows and passengers fled down a slipway, some still hauling their bags.

Many of the dead and wounded at the airport were badly injured in the legs, one airport worker told Reuters, suggesting at least one bomb in a bag on the floor.

The blast hit the train as it left Maalbeek station, close to European Union institutions, heading to the city centre.

​Brian Carroll, 31, a communications consultant from Washington, said he was on a subway car near Maalbeek en route to a conference in downtown Brussels when he heard a loud blast.

"As we were pulling into the station there was suddenly a loud explosion," he said in a phone interview. "There was smoke everywhere. Everyone dropped to the ground. People were screaming and crying."

Carroll said he had remained on the ground for one or two minutes, then got up, pried open a door of the subway car with his hands, and fled.

"I thought to myself, 'I've got to get out of here,'" he said. "I headed toward an exit. There was smoke and soot everywhere. There was glass everywhere. It was like running through a cloud of dust. I saw the exit of the station was destroyed. I ran out of the station, I ran as far as I could."​

VRT carried a photograph of a metro carriage at a platform with doors and windows completely blown out, its structure deformed and interior mangled and charred.

A local journalist tweeted a photograph of a person lying covered in blood among smoke outside the station, on the main Rue de la Loi avenue, which connects central Brussels with the EU institutions. Ambulances were ferrying the wounded away and sirens rang out across the area.

Britain, Germany, France and the Netherlands, all wary of spillover from conflict in Syria, were among states announcing extra security measures. Security was tightened at the Dutch border with Belgium.

World leaders react

"We are at war and we have been subjected to acts of war in Europe for the last few months," French Prime Minister Manuel Valls said.

Prime Minister Malcolm Turnbull tweeted his support for the people of Belgium. He said he was deeply concerned by the attacks and that Australians' thoughts, prayers and solidarity were with the people of Belgium.

The Minister for Foreign Affairs Julie Bishop said: "The Australian Government condemns what appears to be coordinated terror attacks on Belgium. Our thoughts and sympathies are with the people of Belgium."

"DFAT is urgently seeking to determine if any Australians have been affected... I have spoken with our Ambassador Mark Higgie for an assessment of the situation and confirm that our diplomatic staff are safe. If you have any concerns for the welfare of family and friends in the region you should attempt to contact them directly."

"If you are unable to contact them and still hold concerns for their welfare you should call the Department of Foreign Affairs and Trade 24 hour Consular Emergency Centre on 1300 555 135 or +61 2 6261 3305 if calling from overseas".

British Prime Minister David Cameron said he would chair a crisis response meeting following explosions in Brussels on Tuesday.

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"I am shocked and concerned by the events in Brussels. We will do everything we can to help," Cameron said on Twitter, adding that he would chair a meeting of the COBRA response committee.

Reuters, The New York Times and agencies