BRUSSELS (REUTERS, AFP) - A series of apparently coordinated explosions ripped through Brussels airport and a city metro station on Tuesday (March 22), killing at least 34 people in the latest attacks to target Europe.

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, had later blown themselves up in the terminal.

Security was tightened across the jittery continent and transport links paralysed after the bombings that Belgian Prime Minister Charles Michel branded "blind, violent and cowardly".

"This is a day of tragedy, a black day," Michel said on national television.

The Islamic State in Iraq and Syria (ISIS) militant group has claimed responsibility for the attacks.

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

The first two explosions occurred in the departure hall of Brussels airport, killing at least 14 people and injuring more than 30, Belgian media said. Prosecutor Frederic Van Leeuw said there was at least one suicide bomber.

Shortly afterward, an explosion was heard at Maelbeek metro station in Brussels, close to the EU institutions, Belgian broadcaster RTBF reported. At least 20 people were killed in those blasts, and more than a 100 injured, the metro operator and federal prosecutor said.

"According to initial figures, there are around 20 dead with another 106 wounded, 17 of them gravely and another 23 seriously," Local mayor Yvan Mayeur said, describing the scene at Maalbeek station as "very chaotic.

The blasts hit shortly after 8am (3pm Singapore time) and the regional authorities went into emergency mode, with all flights in and out of the airport halted, public broadcaster RTBF reported. Brussels airport will remain closed on Wednesday, the CEO of Brussels Airport Company Arnaud Feist told reporters after ISIS claimed responsibility for attacks.

A third bomb failed to explode at Brussels airport, Lodewijk De Witte, the governor of Flemish Brabant province, told a press conference at the airport, adding that it was later destroyed in a controlled explosion.

Metro operator STIB also announced on Twitter that all metro, bus and tram services have been shut down.

La Libre newspaper reported that two key railway stations in Brussels would reopen at 1500 GMT (11pm Singapore time) to allow for traffic to and from Belgium's capital. However, the central station will remain closed for now, police said. Security at the northern and southern Brussels stations will be beefed up with 500 soldiers, the newspaper reported.

Brussels university VUB also warned its students that it had been alerted of an imminent bomb attack near its campus, which is in the southeast of the city. It tweeted for all students to go indoors and remain there immediately.

Facebook activated its safety check feature to allow people in the area to inform their Facebook contacts that they are safe.

Sky News television's Alex Rossi, who was at the scene, said he heard two "very, very loud explosions".

"I could feel the building move. There was also dust and smoke as well... I went towards where the explosion came from and there were people coming out looking very dazed and shocked."

Jean-Pierre Lebeau, a French passenger who had just arrived from Geneva, told AFP, "We heard the explosion and felt the blowback." He added that he had seen wounded people and "blood in the elevator".

"First we were kept together by the border police, then they gave us the order to evacuate," Mr Lebeau said. The ceilings collapsed, he said, describing a smell of gunpowder at the scene.

With shock on their faces, Mr Jean-Pierre Herman embraced his wife Tankrat Paui Tran, whom he had just gone to get from the airport after her flight from Thailand.

"My wife just arrived," Mr Herman said. "I said hello, we took the elevator and in the elevator we heard the first bomb.

"The second exploded just when we got off. We ran away to an emergency exit. I think we are very lucky."

Brussels Airport has cancelled flights and evacuated the airport area, it said on its Twitter account. The airport will be closed until 6am Wednesday (1pm Singapore time), the airport said. There are no direct flights from Singapore to Brussels.

Brussels Airport serves over 23 million passengers a year. Security has also been reinforced in airports around Europe, including in Paris, London and Frankfurt.

The blasts occurred four days after the arrest in Brussels of a suspected participant in November militant attacks in Paris that killed 130 people.

Belgian Interior Minister Jan Jambon announced that Belgium's terror threat threat had been raised from three to a maximum of four.

He had said on Monday that the country was on the highest level of alert for possible revenge attacks after the capture of Salah Abdeslam, the man believed to be a direct participant in last November's Paris attacks, reported the BBC.

He told Belgian radio: "We know that stopping one cell can... push others into action. We are aware of it in this case."

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.

A bomb and an Islamic State flag were also found on Tuesday in a flat in Brussels, and Van Leeuw confirmed a manhunt was under way.

"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," he told a news conference.

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

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.

Citizens of the United States, Spain and Sweden were among the injured, their governments reported.

The attacks struck at "the whole of Europe", French President Francois Hollande said. "Through the attacks in Brussels, the whole of Europe has been hit," Hollande said in a statement, urging the continent to take "vital steps in the face of the seriousness of the threat.

The Eiffel Tower will be lit up in Belgian colours on Tuesday evening as a symbol of support in Paris for the people of Brussels. "In solidarity with the people of Brussels, Paris will light up the Eiffel Tower this evening in the colours of the Belgian flag," Paris Mayor Anne Hidalgo said on her Twitter account.

There was no immediate official comment on the cause of the attack.

A Slovenian diplomat was injured in the blast in the metro, Slovenian Foreign Minister Karl Erjavec said. Erjavec told a news conference that the diplomat, who he did not name, was in hospital and his life was not in danger.

Belgium's royal palace in central Brussels was evacuated following the blasts, RTBF said. The broadcaster said King Philippe and Queen Mathilde were in shock, but it did not specify whether they had actually been in residence on Tuesday.

Sunni Islam's leading seat of learning, Al-Azhar, said the attacks in Brussels "violate the tolerant teachings of Islam," and urged the international community to confront the "epidemic" of terrorism.

"Al-Azhar strongly condemns these terrorist attacks. These heinous crimes violate the tolerant teachings of Islam," the Cairo-based Al-Azhar said in a statement. "If the international community does not unite to confront this epidemic, the corrupt will not stop from committing heinous crimes against the innocent."