President Barack Obama condemned the "outrageous" attacks Tuesday in Brussels that killed about 35 people, saying the United States would do everything in its power to hunt down those responsible.

"We must be together regardless of nationality or race or faith in fighting against the scourge of terrorism. We can and we will defeat those who threaten the safety and security of people all around the world," he said, speaking in the Cuban capital Havana.

"The thoughts and the prayers of the American people are with the people of Belgium and we stand in solidarity with them in condemning these outrageous attacks against innocent people," said Obama.

"We will do whatever is necessary to support our friend and ally Belgium in bringing to justice those who are responsible, and this is yet another reminder that the world must unite."

A series of explosions ripped through Brussels airport and a metro train, killing around 35 people and injuring more than 200 in the latest attacks to rock Europe.