People leave the scene of explosions at the airport. Twenty more were killed in a subway train blast during rush hour. [Photo/Agencies]

Michel said he was sending up to 200 extra troops to the capital to join those already deployed.

Video showed devastation in the hall at the airport, with ceiling tiles and glass scattered across the floor. Some passengers emerged from the terminal with blood spattered over their clothes.

Britain, Germany, France and the Netherlands, all wary of spillover from the conflict in Syria, were among states announcing extra security measures.

All public transportation in Brussels was shut down.

Authorities appealed to citizens not to use overloaded telephone networks. Additionally, extra troops were sent into the city, and the Belgian Crisis Center, wary of a further incident, appealed to the population: "Stay where you are."

Li Wei, an expert of anti-terrorism studies at the China Institutes of Contemporary International Relations, said "there is no doubt" that the explosions in Brussels were terrorist attacks.

He said the blasts bear resemblance to terrorist attacks in London in July 2005 and in Madrid in March 2004, with both targeting densely populated public transportation venues.

Li said it was likely that terrorists linked to Abdeslam, the man arrested on Friday, were behind the latest attacks.

The Brussels airport said it had canceled all flights until at least 6 am local time on Wednesday, and the complex had been evacuated and trains to the airport halted.

Hainan Airlines' flight HU492, which was scheduled to take off at 12:20 pm, was delayed, the airline said, without disclosing a new flight schedule.

The carrier's Airbus 330 was parking at the airport as it was closed.