World leaders expressed shock and denunciations after the attacks in Brussels on Tuesday that killed more than 30 people and wounded about 200 others.
Two explosions struck Brussels' Zaventem airport, killing 14 people, while blasts also rocked the Maelbeek metro station in the centre of the Belgian capital, adding another 20 to the death toll that is likely to rise with many critically wounded.
The assaults follow November's brazen attacks in Paris that killed 130 people, which the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group claimed responsibilty for.
"We are at war. We have been subjected for the last few months in Europe to acts of war," said France's Prime Minister Manuel Valls.
US President Barack Obama, who is currently in Cuba, called for nations around the world to unite to defeat terrorism.
"We will do whatever is necessary to bring justice to those who are responsible," Obama said. "We must unite. We must be together in fighting against terrorism."
READ MORE: Europe on high alert after deadly Brussels attacks
The attacks come three days after Salah Abdeslam, the most wanted suspect for the Paris attacks, was arrested in a shootout with police in the Belgian capital.
"We realise we face a tragic moment, we have to be calm and show solidarity," said Belgian Prime Minister Charles Michel.
A statement from Belgium's royal family said it was "devastated by the attacks", and the "thoughts of the King and Queen go first to the victims and their families and the rescue services, which are doing everything possible to help the victims".
US Democratic presidential candidate Hillary Clinton said despite Tuesday's carnage, acts of violence "will not succeed", and she vowed to stand by the United States' European allies.
Meanwhile, French President Francois Hollande said the whole world was concerned by the events that unfolded in Brussels.
"Terrorists struck Brussels but it was Europe that was targeted," said Hollande. "All the world is concerned."
Heightened security measures were imposed around the world, with Spain and the Czech Republic deploying more forces in transportation centres.
New York City also added more security personnel to its main train station, and Israel briefly announced that all Israeli flights from Europe were cancelled, but later reinstated those.
"Terrorists will never win" was what Peter Altmaier, chief of staff for German Chancellor Angela Merkel, said, while Turkish Prime Minister Ahmet Davutoglu said the attacks "have once again shown terrorism's global face".
Pope Francis expressed his "deepest sympathy" to those affected by the deadly bombings in Brussels.
"His Holiness Pope Francis entrusts to God's mercy those who died and he prays for those who have lost relatives," a statement said.
One prominent Russian politician highlighted the assaults while saying they showed the need for Western powers to work with Moscow against armed groups.
"It's time for Europe to understand where the genuine threat is coming from and join efforts with Russia," lawmaker Alexei Pushkov said.
Source: Al Jazeera and agencies