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."
The explosions reignited terror concerns only four days after police captured the most-wanted suspect in last year's Paris terrorist attacks, which killed 130 people. Deputy Prime Minister Didier Reynders said no information suggested the attack was linked to the arrest of the suspect, Salah Abdeslam, who was captured in Brussels.
The so-called Islamic State claimed responsibility for Tuesday's attack, according to a post by affiliated Amaq Agency. U.S. government agencies said the terrorist group's claim appears authentic, Reuters reported, citing U.S. officials.
The Islamic State group subsequently issued an updated communique threatening other countries taking part in the anti-ISIS coalition.
The statement promised "dark days" for countries allied against the Islamic State, threatening that "what is coming is worse and more bitter."
But Belgian Federal Prosecutor Frederic Van Leeuw said it was too early to link the bombings to the Paris attacks.
Attackers set off two bombs, which contained nails, at the Zaventem airport, sending passengers fleeing, NBC News reported. Belgian news agency Belga said shots were fired and Arabic was shouted before the blasts, Reuters said.
Photos from the scene showed bloodied victims, smoke, shattered windows and debris. Security services destroyed a third bomb found there. The explosives were contained in suitcases, NBC News reported, citing counterterrorism officials.
Zaventem airport was evacuated, all flights were suspended and trains to the facility were suspended, VRT said. Arnaud Feist, the CEO of Brussels Airport Co., said the airport will remain closed on Wednesday.
The U.S. State Department Tuesday issued a travel alert for citizens travelling to Europe following Tuesday's attacks.
The travel alert, which expires on June 20, advised U.S. citizens to exercise vigilance when in public places or using public transportation.
Federal police confirmed to NBC that the three men in the photo below are considered suspects in the airport bombing:
Police issued a wanted notice for a possible suspect, issuing a photo of a man dressed in white, wearing a hat and pushing a luggage cart. Van Leeuw said the other two suspects likely blew themselves up at the airport. He added raids are taking place across the country.
Belgian police are searching a residence that may have served as a staging ground for the attack, a senior U.S. intelligence official told NBC News. Belgian authorities said they found chemical products, an Islamic State flag and an explosive containing nails during a raid in the Brussels neighborhood of Schaerbeek, according to the AP.
After the blast at the metro station of Maelbeek, close to the European Union institutions in central Brussels, the city's public transport authority said all metro stations were closed.
By about 4:25 p.m. local time, the Belgian crisis center said people could move around the city again and metro stations would start reopening despite a continued threat, according to The Associated Press.
Charlotte Van Driessche, who was on the metro train behind the one under attack, told public broadcaster VRT: "We heard a loud bang. … There was chaos immediately as everyone suspected the attack was linked to the blast in Zaventem. … There was a huge amount of panic. ... There was a terrible smell, a real smell of burning."
The Belgian ministry of health said the airport bombing killed at least 11 people and injured 81. Yvan Mayeur, the mayor of Brussels, said 20 people were believed dead and 106 injured in the metro bombing. Officials are still trying to determine whether another attack is imminent.
Mourners gathered in Brussels on Tuesday night, laying flowers and candles around makeshift memorials.
Speaking in Cuba on Tuesday morning, President Barack Obama pledged to support Belgium in bringing the attackers to justice.
"We stand in solidarity with them, condemning these outrageous attacks against innocent people," he said.
Obama spoke with Michel, offering assistance in investigating the attacks, the White House said.
The Obama administration is expected to announce new airport security measures in response to the attacks, Reuters reported, citing a government source. The Department of Homeland Security separately said the Transportation Security Administration would send additional security to major U.S. airports as a precaution.
VRT said at least one of the Brussels airport explosions happened near the check-in desks for American Airlines, but the airline later disputed that report. The company told CNBC in an emailed statement it had accounted for all of its employees and contractors with no reported injuries.
Starbucks said initial indications showed a blast went off near one of its stores in the airport. The company said one employee was injured and it has accounted for all other workers.
The U.S. Air Force confirmed an Airman and four of his family members were injured in the airport attack. The Air Force did not identify the man, his family members or the nature of their injuries.
The Mormon church also said three missionaries were injured at the airport. They were listed in critical condition.
Belgium has been on high terror alert since the Paris attacks on Nov. 13.
Former French Prime Minister Dominique de Villepin told CNBC that the attacks were "tragic events" but added that Europe could only "reduce" the threat of terrorism.
"I do believe that our strategy should be very different than the one it is. Much less military approach than a political approach, trying to find solutions in the Middle East, and we are far from doing that," he said.
— NBC News, Reuters and The Associated Press contributed to this report.