Islamic State claimed responsibility for attacks on Brussels airport and a rush-hour metro train in the Belgian capital on Tuesday which killed at least 30 people, a news agency affiliated to the group said.
The Amaq agency said suicide bombers from the group, strapped into explosive belts, had staged both attacks. Belgian media said police were hunting one attacker who had survived.
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.
A witness said he heard shouts in Arabic and shots shortly before two blasts struck a packed airport departure lounge at Brussels airport.
Belgian media published a security camera picture of three young men pushing laden luggage trolleys through the airport and reported that police suspected them of being the attackers. They said two were suspected of having blown themselves up while police were hunting the third.
Amaq carried the claim of responsibility several hours after the morning attacks. "Islamic State fighters opened fire inside Zaventem Airport, before several of them detonated their explosive belts, as a martyrdom bomber detonated his explosive belt in the Maalbeek metro station," it said.
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.
U.S. President Barack Obama led calls of support to Belgian Prime Minister Charles Michel after Brussels went into a state of virtual lock-down.
"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."
Michel spoke at a Brussels news conference of a "black moment" for his country. "What we had feared has come to pass."
The blasts occurred after the arrest in Brussels of a suspected participant the Paris attacks that killed 130 people. Belgian police and combat troops on the streets had been on alert for reprisal but the attacks took place in crowded areas where people and bags are not searched.
All public transport in Brussels was shut down, as it was in London during 2005 Islamist militant attacks there that killed 52. Authorities appealed to citizens not to use overloaded telephone networks, extra troops were sent into the city and the Belgian Crisis Centre, clearly wary of a further incident, appealed to the population: "Stay where you are".
Brussels airport will remain closed on Wednesday, its chief executive Arnaud Feist told reporters.
Public broadcaster VRT said police had found a Kalashnikov assault rifle next to the body of an attacker at the airport. Such weapons have become a trademark of Islamic State-inspired attacks in Europe, notably in Belgium and France, including on Nov. 13 in Paris.
An unused explosive belt was also found in the area, the public broadcaster said. Police were continuing to scour the airport for any further bombs or attackers.
Alphonse Youla, 40, who works at the airport, told Reuters he heard a man shouting out in Arabic before the first explosion. "Then the glass ceiling of the airport collapsed."
"I helped carry out five people dead, their legs destroyed," he said, his hands covered in blood.
A witness said the blasts occurred at a check-in desk.
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.