"It is probably a gas explosion," an official at the agency told Reuters, adding that there was no indication of it being a militant attack.
Local media reported the blast occurred just after midnight 03:30 IST at a sports centre known as "Le Chalon" in Chimay, close to the French border, when five people were still inside the building. Part of the building was destroyed.
Belgium and France, along with the rest of Europe, have been on high alert after Islamic State attacks in Paris and Brussels over the past year.
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