AGF, the French insurance arm of Germany's Allianz, has forecast total loss exposure caused by the explosion at a chemical factory in Toulouse at between euros 610m and euros 910m (£378m and £564m).
The company is the lead insurer of the factory, which is owned by petroleum giant TotalFinaElf. It said it expected its own exposure to be in the $7m (£4.7m) range.
It said its primary exposure of around $422m (£287.1m) would be offset by its reinsurance coverage.
