Belgian authorities are cautioning that a suspect may be on the loose after deadly back-to-back bombings stunned Brussels on Tuesday, moving the country to its highest level of alert and spreading fear across Europe.

The Islamic State group has claimed responsibility for a pair of bombs that exploded around 8 a.m. local time in the departures area of the Zaventem airport, as crowds were checking in for morning flights. About an hour later, another blast struck morning commuters at the Maelbeek subway station in central Brussels, located near the European Union headquarters.

At least 30 people were killed and more than 200 wounded in the attacks as a European capital was again locked down amid heightened security threats.

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.

Belgium raised its terror alert to level four — which signifies a "serious and imminent attack" is likely — diverting planes and trains. People were ordered to stay where they were for most of the workday and a police search operation was continuing into the night.

Adrienne Arsenault looks at why Belgium is a target for an attack. 3:40

At Belgium's request, EU ministers will meet to discuss the attacks, Dutch Justice Minister Ard van der Steur announced on Twitter. The Netherlands, which currently holds the European Union presidency, will organize the event. It may take place as early as Thursday morning.

Earlier Tuesday, Belgian Prime Minister Charles Michel declared three days of national mourning.

"What we feared has happened," Michel said. "In this time of tragedy, this black moment for our country, I appeal to everyone to remain calm but also to show solidarity."

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

Suspects' photo released

In the wake of the chaos, authorities in Brussels released a photo from security footage of three suspects in the airport attacks.

The photo shows three men pushing baggage trollies. The two men with dark hair — wearing gloves only on their left hands — are believed to have been suicide bombers. A third man — dressed in a light-coloured coat, black hat and glasses — is being sought by police, who issued a wanted notice and urged the public to call if they recognized him.

This CCTV image shows three men who are suspected of taking part in the attacks at Belgium's Zaventem Airport on March 22, 2016. The man at right is still being sought by the police, while the two others were 'probably' suicide bombers, according to a Belgian prosecutor. (Belgian Federal Police/Associated Press)

The Zaventem airport was ordered closed and will remain shuttered at least through to the end of Wednesday.

Authorities found and neutralized a third bomb "with a controlled action" at the airport once the chaos had eased, airport spokesperson Florence Muls told The Associated Press. According to authorities, bomb squads also detonated suspicious objects found in at least two locations elsewhere in Brussels, but neither contained explosives.

The exact number of people killed and injured in the attacks is still unclear because of the carnage at the scenes.

The blasts occurred at two locations in Brussels - at the Zaventum airport and at the Maelbeek subway station. (Reuters)

Explosive device, chemicals, flag found

Belgium's federal prosecutor Frederic Van Leeuw, who described the explosions as terror attacks, said two men "probably" staged suicide bombings at the airport and a third fled.

Prosecutors also said a home search in the neighbourhood of Schaerbeek "led to the discovery of an explosive device containing, among other things, nails." Investigators also found chemical products and an Islamic State flag.

Reuters reported that a witness said he heard shouts in Arabic shortly before two blasts struck the packed airport departures area. Airport officials said police discovered a Kalashnikov assault rifle and an explosives-packed vest abandoned at the facility, offering one potential lead for forensic evidence.

In its claim of responsibility, posted on its blog and Telegram account, ISIS said its members detonated suicide vests both at the airport and in the subway.

This was the scene a short while ago, between the Arts-Lois and Maelbeek metro stations in Brussels. <a href="https://t.co/aTZjqsF7Gt">pic.twitter.com/aTZjqsF7Gt</a> —@evanlamos

The explosions came just days after the main suspect in the Paris attacks was arrested in Brussels. After his arrest, Salah Abdeslam told authorities he had created a new network and was planning new attacks.

But Belgium's federal prosecutor had cautioned it was too early to link the bombings to the Nov. 13 Paris attacks that killed 130.

The two airport explosions splattered blood across the departure lounge and collapsed the ceiling. Witnesses told The Associated Press that one occurred at an excess baggage payment counter and the other near a Starbucks café.

An injured woman leaves the scene at Zaventem airport near Brussels after explosions rocked the facility. (David Crunelle/Associated Press)

All flights were cancelled and arriving planes and trains were diverted. Authorities told people in Brussels to stay where they were, bringing the city to a standstill, as security at the borders was tightened.

Airport security was also tightened in Paris, London and other European cities.

European security officials have been braced for a major attack for weeks and warned that ISIS was actively preparing to strike. Abdeslam's arrest on Friday heightened those fears, as investigators said many more people were involved in Paris than originally thought, and that some are still on the loose.

Jan Jambon, Belgium's interior minister, said authorities knew some kind of extremist act was being prepared in Europe but that they were surprised by the scale of the attacks in Brussels.

"We had no information about this, but we know that things were moving in Europe, in different countries, in France, in Germany, here," he told RTL Television.

People display a solidarity banner in Brussels following Tuesday's bomb attacks. The banner reads 'I am Brussels' in French and in Flemish. (Charles Platiau/Reuters)

Witnesses describe 'war scene'

Passenger Cedric Vanderswalm said a late train and a full elevator at the airport probably saved his life.

The 20-year-old from the coastal Belgian town of Knokke was at the Brussels airport on Tuesday planning to fly to London for his job as an animator.

He said was heading to the airport's departures level but the elevator was full "so I didn't get in. I waited and I was about to step into the elevator when there was a big explosion." He said people started running, dropping their luggage.

He said "if I had taken the previous elevator, I would have been right in the explosion. My train also had a five-minute delay, so I was lucky."

The explosion coated the left side of his face with soot and dust.

Zach Mouzoun, who arrived on a flight from Geneva about 10 minutes before the first blast, told BFM television that the second, louder explosion brought down ceilings and ruptured pipes, mixing water with victims' blood.

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"It was atrocious. The ceilings collapsed," he said. "There was blood everywhere, injured people, bags everywhere.

"We were walking in the debris. It was a war scene," he said.

Julian Firkins, a British man who's been living in Brussels for two decades, had just finished dropping off his girlfriend at the airport when he heard the first bomb go off. He had gone to a nearby coffee shop to grab a drink.

"I immediately turned around and ran straight back to my girlfriend, who had dived on the floor. I dived on top of her and pulled her suitcase over both of us just to protect us from falling debris," Firkins told CBC's As it Happens. "There were just people running around and stuff was falling from the ceiling. The windows had been blown in as well."

Firkins and his girlfriend — who were not injured — got up when airport staff told everyone to run toward the exits. Once outside, Firkins said he saw many injured people, sitting on benches or lying on the ground.

"My girlfriend and I helped a woman who had a serious injury on her leg. It was shrapnel embedded in her foot ... She was screaming at us to take her shoe off," said Firkins. "But we could see very clearly that we shouldn't because it was the only thing holding her foot together."

With three runways in the shape of a "Z," the Zaventem airport connects Europe's capital to 226 destinations around the world and handled nearly 23.5 million passengers in 2015.

An injured man lies on the ground at the airport. The Brussels prosecutor's office called the explosions at the airport and the metro station terror attacks. (Ketevan Kardava/Georgian Public Broadcaster/Associated Press)

Meanwhile, near the entrance to the Maelbeek subway station, rescue workers set up a makeshift treatment centre in a pub. Dazed and shocked morning travellers streamed from the metro entrances as police tried to set up a security cordon.

"The Metro was leaving Maelbeek station for Schuman when there was a really loud explosion," said Alexandre Brans, 32, wiping blood from his face. "It was panic everywhere. There were a lot of people in the metro."

Francoise Ledune, a spokeswoman for the Brussels Metro, said on BFM television that there appeared to have been just one explosion, in a car that was stopped at Maelbeek.

Shock waves from the attacks also crossed the Atlantic, where officials in several U.S. and Canadian cities increased security at airports and other locations.

Canada offers condolences

Prime Minister Justin Trudeau and Foreign Affairs Minister Stéphane Dion both offered condolences Tuesday on behalf of Canada.

"This is a black Tuesday for Belgium," said Dion, speaking at a news conference on Parliament Hill in Ottawa. "Hearing the screams of children in the smoke of the Brussels metro only strengthens our resolve to combat terrorism in all its forms and increases our solidarity with the people of Belgium in the whole of Europe."

Trudeau told reporters he was "outraged" to hear of the attacks.

"What happened today in Brussels was an act of terror. It was violence directed at innocent civilians and its goal was to take lives and instil fear. This cannot and will not be tolerated," he said.

I strongly condemn today's deplorable terrorist attacks in Brussels. My thoughts are with the victims as we stand with Belgium & the EU. —@JustinTrudeau

Raoul Delcorde, Belgium's ambassador to Canada, thanked Canada for its support and called for a "strong response" from both the European and international communities.

"A country alone cannot effectively fight this scourge and we know that nothing must prevent us from maintaining our values," Delcorde said.

A media relations person for Global Affairs Canada said they are not aware of any Canadian casualties in the explosions.

Canadians in <a href="https://twitter.com/hashtag/Belgium?src=hash">#Belgium</a>: please consult our latest Travel Advice and Advisories <a href="https://t.co/zplclI6MUT">https://t.co/zplclI6MUT</a> —@TravelGoC

As night fell, some of Europe's best-known monuments — including the Eiffel Tower and the Trevi Fountain — were illuminated with Belgium's national colours.