UPDATED 2.37AM: Islamic State has claimed responsibility for suicide bomb attacks on Brussels airport and a rush-hour metro train in the Belgian capital which killed at least 30 people.

WATCH: Chaos after terror hits Brussels airport

MORE: Kiwis warned against high-risk travel to Belgium

LISTEN: Kiwi in Brussels - 'I saw devastation, panicked faces'

WATCH: Why are Belgians joining the Islamic State?

OPINION: Solidarity against terror is how we will win

Police issued a wanted notice for a young man in a hat who was caught on CCTV pushing a laden luggage trolley at Zaventem airport alongside two others who, investigators said, later blew themselves up in the terminal, killing at least 10 people.

Officials said about 20 died on the train close to European Union institutions. Islamic State said that too was a suicide attack. The tolls were vague because of the carnage at both sites.

Reports say over one hundred people have injured in the attacks.

The coordinated assault triggered security alerts across Europe and drew global expressions of support, four days after Brussels police had captured the prime surviving suspect in Islamic State's attacks on Paris last November in which 130 people were killed.

Belgian authorities were still checking whether the attacks were linked to the arrest of Salah Abdeslam, Federal Prosecutor Frederic Van Leeuw said, although Belgian security experts said the level of organisation involved suggested they had been in preparation for more than just a few days.

As the city began to emerge from a day of lockdown, a major police search operation was still under way after dark in the northern borough of Schaerbeek.

Investigators said they had found a nail bomb and an Islamic State flag in an apartment.

Private broadcaster VTM said police went to the area after a taxi driver reported driving three people to the airport and became suspicious when they did not let him touch their baggage.

Last week, explosives and an Islamic flag were found during a raid on an apartment in the south of Brussels.

Police also found a fresh fingerprint of Abdeslam's there, putting them on to his trail. It was not clear if Abdeslam had been involved at that stage in planning the airport attack.

In a statement, Islamic State said "caliphate soldiers, strapped with suicide vests and carrying explosive devices and machine guns" had targeted the airport and metro station, adding that they had set off their vests amidst the crowds.

It was not clear, however, that the attackers used vests.

The suspects were photographed pushing bags on trolleys and witnesses said many of the airport dead and wounded were hit mostly in the legs, possibly indicating blasts at floor level.

"A photograph of three male suspects was taken at Zaventem.

Two of them seem to have committed suicide attacks. The third, wearing a light-coloured jacket and a hat, is actively being sought," prosecutor Leeuw told a news conference.

A government official said the third suspect had been seen running away from the airport building. Police later found and detonated a third explosive device at the airport.

Belgian police appealed to travellers who had been at the airport and metro station to send in any photographs taken before the attacks in their efforts to identify the bombers.

After questioning Abdeslam, police issued a wanted notice on Monday, identifying 25-year-old Najim Laachraoui as linked to the Paris attacks. The poor quality of Tuesday's CCTV images and of the Laachraoui wanted poster left open whether he might be the person caught on the airport cameras.

A lockdown imposed after the attacks was eased and commuters and students headed home as public transport partially reopened.

services on the cross-channel tunnel from London to Brussels were suspended.