The Mayor of Reims Adeline Hazan has told the French news agency BFM TV that at least "two people were killed" in the accident at a four storey block of flats in the city.
Initial reports on local French media suggest the explosion may have been caused by a gas leak.
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The father-of-four from Cardiff with appear in court this afternoon charged with terrorism-related murder and attempted murder.
Loyalist paramilitary commander turned supergrass Gary Haggarty has pleaded guilty to 200 terrorist offences, including five murders.
European Commission President Jean-Claude Juncker says the Prime Minister's 'fair and serious' offer to EU nationals doesn't go far enough.