World leaders have expressed their shock at the attacks in Brussels. Here are some of the reactions:

"We are at war. We have been subjected for the last few months in Europe to acts of war." - French Prime Minister Manuel Valls.

"We realise we face a tragic moment. We have to be calm and show solidarity." - Belgian Prime Minister Charles Michel.

"The King and Queen are devastated by the attacks." - statement from Belgium's royal family.

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"These attacks mark another low by the terrorists in the service of hatred and violence." - EU Council President Donald Tusk.

"Terrorists struck Brussels but it was Europe that was targeted, and all the world that is concerned."- French President Francois Hollande.

"Terrorists will never win."- Peter Altmaier, chief of staff for German Chancellor Angela Merkel.

"It is with a heavy heart we learned of these ruthless attacks in Brussels ... The world must stand united to defeat terrorism." - Salem Al Meslet, of High Negotiations Committee, a coalition of Syrian opposition groups engaged in peace talks in Geneva.

The attacks "have once again shown terrorism's global face". - Turkish Prime Minister Ahmet Davutoglu.

"It's time for Europe to understand where the genuine threat is coming from and join efforts with Russia." - Prominent Russian lawmaker Alexei Pushkov.