A man comforts a woman outside Brussels Airport after explosions rocked the departure lounge, killing at least 14 people on Tuesday. [Photo/Agencies]

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.