Several buildings were set ablaze on both sides of a street in the seaside town of St Laurent-de-la-Salanque after a blast was heard at 01:30 local time (00:30 GMT) on Monday.
Firefighters are still searching for victims in two small blocks of flats damaged by the fire.
The cause of the original explosion is not yet known.
France's Interior Minister Gérald Darmanin, who visited the scene of the accident on Monday afternoon, said the current death toll of seven was only provisional while emergency services continued to search for survivors.
"Prosecutors currently don't have any crucial information to determine whether this was accidental or criminal," he added.