European countries have responded to the attacks in Brussels by moving quickly to increase their own security measures.
France's Interior Minister Bernard Cazeneuve said his country would deploy 1600 security officers in response to the attacks.
The announcement follows an alert that was issued at the Gare du Nord train station in Paris, from where trains travel to and from Brussels.
The Wall Street Journal quoted an official from SNCF, the French state-owned rail company, as saying: "We have sent more security agents to the Gare du Nord train station."
France was already in a high state of alert, carried over from the Paris attacks in November, in which 130 people were killed.
Denmark, Sweden and Finland have stepped up security at airports and in public places, and Britain also said it would be increasing security following the explosions in Brussels.
Earlier, British Prime Minister David Cameron announced he would chair a meeting of the emergency committee known as COBRA.
Newshub UK correspondent Tova O'Brien said the increased police presence was already visible in London.
The German Federal Police announced it was also increasing security, Al Jazeera reported.