More than 30 people were killed in explosions at Zaventem global airport and Maelbeek metro station in the centre of Brussels on Tuesday morning.

The entire Brussels public transport system was closed early Tuesday, but select metro, tram and bus lines have resumed service, according to the system's web site.

The San Francisco Police Department has deployed extra officers at San Francisco International Airport, the Port of San Francisco, and at San Francisco Municipal Railway stations following the explosions at Brussels Airport and the Maalbeek metro station in Brussels, police spokesman Officer Albie Esparza said.

Brussels Airport CEO Arnaud Feist told ABC News all flights and operations were canceled for Tuesday until at least Wednesday, and the facility has been evacuated. The explosion at the metro station was close to buildings that house European Union institutions. North Atlantic Treaty Organisation is also headquartered in Brussels. "Two of them have probably committed a suicide attack, the third - dressed in a white jacket and wearing a hat - is actively sought". Belgian public broadcaster IRTV was reporting a total of 34 dead, but that number could not be immediately verified. The names are being run through databases of suspected or known terrorists.

"These attacks normally take a fair bit of preparation, in terms of reconnaissance, explosives preparations, coordination", said Matthew Henman, head of the IHS Jane's Terrorism and Insurgency Centre in London.

"Three bombs were brought into the building, of which one failed to explode", Lodewijk De Witte, the governor of Flemish Brabant province, told a press conference at the airport.

Herman embraced his wife Tankrat Paui Tran, who he had just gone to collect from the airport after her flight from Thailand. The Belgian federal prosecutor said suicide bombers likely were involved.

Investigators were searching residences, but they weren't saying where, Van Leeuw said.

The bomb was reportedly detonated in the middle carriage, which was running along the platform at the time.

Benzema isn't the only one who was shocked to discover that Abdeslam had been hiding out in Brussels.

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Social media showed pictures of smoke rising from the departure hall where windows had been shattered by the blasts.

The airport evacuation left hundreds of people stuck on the parking lot and on the runway of the airport.

A photo of Zaventem airport in Brussels, Beligum, following a Tuesday morning terrorist attack. The British newspaper said the meeting may take place as soon as Thursday.

So-called ISIS or Daesh has claimed responsibility for the attacks, it has been widely reported.

People in Brussels have been asked not to travel today. Many have been returning.

The city was already on high alert after the arrest of Salah Abdeslam, a key suspect in the Paris terror attacks in November that were claimed by Daesh. In the aftermath of the attacks, raids have been carried out around Belgium.

Brussels police called in a statement late Tuesday for help from "anyone who has amateur film where the attackers may be in view and could help move their investigation forward". The group identified Belgium as "a country participating in the worldwide coalition against the Islamic State". An ISIS flag was found during the raid.

Authorities appear to have focused predominantly on Molenbeek, an inner-city suburb in Brussels that has always been linked to terror and portrayed as a jihadi breeding ground. They found bomb-making materials in one of them.

Obama addressed the attacks while he was in Cuba for the final day of his groundbreaking visit to the United States's former Cold War foe.

"This is yet another reminder the world must unite".