Half-masted European Union flag is seen on the roof of the European Parliament information office in Helsinki, Finalnd 22 March, 2016.[Photo/Agencies]

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.

The subway blast hit the train as it left Maelbeek station, close to European Union institutions, heading to the city center.

European governments moved swiftly to beef up security in the wake of the Brussels attacks.

In France, President Francois Hollande ordered an additional 1,600 police and paramilitary officers onto the streets of Paris.

"This is a war against terrorists," Hollande told reporters. "It will be a long war, but we should be calm and determined.

"Today we are with Belgium," he added.

Wang Qingyun and Wang Wen in Beijing, and Chris Peterson in London contributed to this story.