Belgian officials said the bombings at Zaventem airport and the Maelbeek metro station killed at least 34 people and injured more than 230, according to reports. Officials raised the city's terror threat level and shut down public transit after the blasts.
Belgian Prime Minister Charles Michel called Tuesday a dark moment for the country.
"What we feared has come to pass. Our country has been struck by attacks which are blind, violent and cowardly," Michel told a news conference.
Interior Minister Jan Jambon told NBC News that authorities were now in a high state of alert and working "day and night to arrest these guys."
"The people of IS are also professionals. So that means that the job is extremely difficult and we are doing everything to make it to a success."